Prime contender

Prominent beyond mention (in soaking sense) is the little town of Loutra Aidipsos (Edipsou / Edipsos) on the Greek island of Evia (Euboea) which is regarded as one of the nations premier hot spring resorts.

With this reputation comes a tad of tackiness: there's little the town can endear itself except a plethora of hot spring related / tourist business. 
And there's a lot of this, tripadvisor alone refers you to more than 100 accommodation options. Add to that a plethora of rental apartments. 
And you'll have to conclude that soaking is a way of life in Aidipsos.

Mint
And why is soaking so popular? 

One aspect would be it's vicinity to Athens. In a way it's far away from the capital yet not so poorly accessible especially figuring in the ferry after which it's just a 1,5 hours car drive to Athens.

Ferrying to the mainland, just half an hour but a step back from the hustle and bustle.

 
Then the number of hot springs themselves: 80 of Greece's 752 hot springs are located in Aidipsos according to wikipedia. Which is lot. So a lot of choice. Many are also on the seaside, so what better than have a peaceful holiday on the beach and soak at the same time? 
And with the visitor having a choice of where to stay and what soak to enjoy comes a distinctive turn away from the general Greek (Mediterranean?) penchant for medicinal soaking shunning the fun.

Then there's the historic edge. Matt has this:

'Edipsos is not a new phenomenon. Even though few travelers from other countries have never heard of  it the cosmopolitan atmosphere combined with the healing spas have attracted reknown politicians, artists, writers and other notable people like Winston Churchill, Aristotle Onnasis, Maria Callas, Greta Garbo and Omar Sharif. But it is mentioned in the works of Aristotle in his Meteorological, and by Ploutarch and Strabon. The town even minted its own coins. In Roman times the area flourished and its healing waters were visited by the emperors Hadrian, Septimus Severus and Marcus Aurilius. The baths from the Roman period are the best preserved and are known as Syllas Baths. During the Byzantine era it was destroyed for being an area of paganism though it was visited by the Emperors Theodosious and Constantine the Great'.

So well connected thus, historically speaking.

Disorder
It's quite obvious that soaking has always been an attraction here. And still is. 

Tripadvisor rates Aidopsos soaks with 4 stars (based on 55 reviews), mostly an average review from a diverse range of nationalities.  
Tripadvisor also ranks the hot springs as the number 5 of things to do on Evia island, itself the second largest island of Greece.

But let's look at more detail into the springs themselves. This website notes:

The hot springs of Edipsos are known since the ancient times. In the archaeological collection of the city, the visitor has the opportunity to see important ancient baths remains dating from the Hellenistic and Roman period. A large modern complex of hydrotherapy, including steam baths and Jacuzzi, operates in Edipsos. It has an outdoor Olympic-sized pool which waters come both from the hot springs and from the sea, as well as two indoor pools of thermal water. The hydrotherapy center does not have a hotel. Hence, the visitor must book their rooms in a lodging option in the area. These waters are said to treat acute and chronic rheumatism, arthritis deformities, espondilartritis, neuritis, sciatica, lumbago and injuries caused by ankylosis, gynecological disorders (salpingitis, endometrial infections, ovaries disorders, leucorreas and even some forms of infertility). Source: www.greeka.com

'The hot springs of Edipsos are known since the ancient times. In the archaeological collection of the city, the visitor has the opportunity to see important ancient baths remains dating from the Hellenistic and Roman period. A large modern complex of hydrotherapy, including steam baths and Jacuzzi, operates in Edipsos. It has an outdoor Olympic-sized pool which waters come both from the hot springs and from the sea, as well as two indoor pools of thermal water. The hydrotherapy center does not have a hotel. Hence, the visitor must book their rooms in a lodging option in the area. These waters are said to treat acute and chronic rheumatism, arthritis deformities, espondilartritis, neuritis, sciatica, lumbago and injuries caused by ankylosis, gynecological disorders (salpingitis, endometrial infections, ovaries disorders, leucorreas and even some forms of infertility)'.

The hot springs of Edipsos are known since the ancient times. In the archaeological collection of the city, the visitor has the opportunity to see important ancient baths remains dating from the Hellenistic and Roman period. A large modern complex of hydrotherapy, including steam baths and Jacuzzi, operates in Edipsos. It has an outdoor Olympic-sized pool which waters come both from the hot springs and from the sea, as well as two indoor pools of thermal water. The hydrotherapy center does not have a hotel. Hence, the visitor must book their rooms in a lodging option in the area. These waters are said to treat acute and chronic rheumatism, arthritis deformities, espondilartritis, neuritis, sciatica, lumbago and injuries caused by ankylosis, gynecological disorders (salpingitis, endometrial infections, ovaries disorders, leucorreas and even some forms of infertility). Source: www.greeka.com

The hot springs of Edipsos are known since the ancient times. In the archaeological collection of the city, the visitor has the opportunity to see important ancient baths remains dating from the Hellenistic and Roman period. A large modern complex of hydrotherapy, including steam baths and Jacuzzi, operates in Edipsos. It has an outdoor Olympic-sized pool which waters come both from the hot springs and from the sea, as well as two indoor pools of thermal water. The hydrotherapy center does not have a hotel. Hence, the visitor must book their rooms in a lodging option in the area. These waters are said to treat acute and chronic rheumatism, arthritis deformities, espondilartritis, neuritis, sciatica, lumbago and injuries caused by ankylosis, gynecological disorders (salpingitis, endometrial infections, ovaries disorders, leucorreas and even some forms of infertility). Source: www.greeka.com

Then there's this link:

'Edipsos is located at the green Northern part of Evia and is one of those unique places in Greece, where one can combine Natural Traditional Therapy, the latest in Thermal Spring Therapy with the beaches and atmosphere of a Greek island, the cosmopolitan atmosphere combined with the healing spas. There are more than 80 individual hot-water-springs ranging from 28 – 86 degrees centigrade, that can be very effective in curing problems like rheumatoid and inflammatory arthritis, spondylo- arthritis, tendonitis and many more'.

It's quite odd that most internet sites out there solely focus on attracting  the tourist; there seems little first-hand experiences.

Findings

We spent two nights in Aidipsos. I had banked on there being free and easily available seaside soaks, that much I had gleamed from internet. 

We arrived late the first day, spending a bit of time finding our digs even though the town plan seems very much straight forward (three long lanes running parallel to the long side of the beach cut through by small crossroads often). 

Along the beach itself is the main drag, mostly restaurants, not much to choose between. 

After dinner we move further to the cape, rounding it which passes the entry to the apparent gem of Loutra Aidipsos: the Thermae Sylla Spa & Wellness Hotel (photo below). It's regarded by some as one of the worlds premier spas (source):

'... one of the top 10 natural spas in the world, as voted by the prestigious Conde Nast Traveller [CNT] magazine'.

one of the top 10 natural spas in the world, as voted by the prestigious Conde Nast Traveller magazine - See more at: http://greece.greekreporter.com/2015/08/28/revitalize-yourself-in-greeces-hottest-spa-resort-in-edipsos-on-euboea-island/#sthash.RZUgkixU.dpuf

It actually was ranked fifth in 2011 in the Readers Awards of CNT category Medical, Thermal and Natural Spas.

 

Continuing: the road follows the cape and just beyond the hotel the beach gives way to 5-10m high cliffs. This would be where the soaks should be, but it seems in the closing of the day pretty much deserted (which in itself is a good sign for natural soaking) but also slightly eery. After the first cliff there's a bathing building which has been closed altogether and certainly has seen better times. There's precious little to soak in, just a trickle here and there. 

Surprisingly there's a building a block back on whose ground seems to give rise to many a hot spring. We circle the building the wrong way, around the back the going gets difficult in the dark. All very strange, back in the light of street we can follow an overflow along a bathing building (closed) and the waters disappear underground. 

How odd all. Are the springs all siphoned off for hotels? Even though there are not so many tourists (probably even less than there are soaks). 

Changes
The next morning we learn. The grounds on which the springs spring belong to the National Tourist Organisation. 

We must have missed the cave behind the building. Belonging to Sylla, wikivoyage has this:

'One of the surviving findings of the Roman period are the baths in "Cave of Sulla." The "cave" Sulla is a small building with a dome. The entire building is covered by deposits of sulfur waters that flow in the area and gives the impression of a cave entrance. At the entrance of the "cave" are two massive pedestals of statues with inscriptions in honor of the Roman Emperor Hadrian and Septimius Severus dedicated by the municipality Istieas. The first bear and later inscription in honor of Emperor Constantine of Byzantium. The "cave" was associated with the Roman general Sulla, who was visiting the spa treatment'.

 Around the corner the bathing building is noted as the

'... old spa of St. Anargyroi, a neoclassical architectural jewel gracing with his presence and reminds times when the resort of Edipsos dominated the Mediterranean'.

We do a similar swirl of town as the evening before. 

First it seems things have changed. Hot and sulphorous reeking water is flowing over the rocks  east of Sylla Spa. Has the tap been turned on (or off somewhere else?). 

Strange, but we conclude that the waters are hot and salty. Unfortunately the tide is in, so there are no appropriate pools to soak in: either boiling hot. Or cold.

Continuing onwards and today's discovery does certainly point to more waters as all along this shore waters are running over rocks and small culverts to sea. But nothing to really soak in. The famous sea based shower doesn't seem to be running either, so we leave it for today, not really over excited and look for a great alternative for today.

Looking back to the cape. The first cliff with a building on top, is what appears to be a shower (sometimes). The yellowish cliff has a couple of pools and in between are the deserted bathing facilities.

Dip

Is there anything else to Loutar Aidipsos? 

Well, the town's beaches are mediocre at best. Head over and beyond the bay for better or head along the south coast for more attractive settings.

There are also a hot spring nearby at Loutra Gialtron.

Head inland for some great scenery, some walks (I hope) but also visit the great Drimona waterfall (below), way up in the mountains. Very deserted on our visit it afforded us with delicious dip.