Kos is the only island of the Dodecanese that has three kinds of mineral hot springs due to its volcanism: warm, lukewarm and cold. Note that none of them is operated as a spa. All of those springs are freely accessible to the public provided that you find a way, a vehicle and a map to get you there.
The most important of them are:
The hot spring of Agios Fokas (Empros Thermes). It flows into the beach of Thermes. It is a thermal mineral spring with water reaching a temperature of 47o C. It has great concentrations of sulfur, potassium, sodium, magnesium and calcium ions and is good for rheumatic diseases, arthritis and gynecological problems.
The hot spring of Agia Irini (Piso Thermes). This spring is close to the previous one and is accessible by boat or by following a trail. It is lukewarm and has concentrations of chlorine and sodium. Its water reaches a temperature of 45o C and it is good for kidney diseases.
The hot spring at Kokkinonero. It is located southwest of the Asclepeion flowing into its archaeological site. The water is ferruginous and rich in carbon oxides at a temperature of 22,5o C. It helps fight off dyspepsia and intestinal ailments.
The Volkanon hot spring. Muddy mineral water that is suitable for mud baths springs from another point very close to the hot spring of Kokkinonero.
Source: V.Hatzivasileiou: History of the island of Kos, 1990 pp. 23-24, www.thermalsprings.gr