The word sounds like a hotel in Brittany, all bathrobes and seaweed wraps. It comes from the ancient Greek "thálassa", the sea, and the idea really is old. In 1750 the English doctor Richard Russell wrote his thesis on the effects of seawater, and in the 19th century the treatment became popular across Western Europe, especially in France. Thalassotherapy still means seawater, sea air, sun, algae, mud and sand.
The idea has since reached the dog salon, where it usually means something simpler: a bath or wrap with marine products such as sea salt, algae or mud. That's fine, as long as you know what you're getting. The word thalasso isn't protected, so it turns up far from any coast. True thalassotherapy, strictly speaking, needs fresh seawater and a place right by the sea, which the mountains simply can't offer.
What does it do for a dog? A little honesty helps here. Providers promise a cleaner coat, less odour and better circulation. In people, medical thalasso is used mainly for skin conditions such as eczema and psoriasis, while in wellness it serves relaxation only. We know of no solid studies showing healing effects in dogs. If you expect a seaweed bath to cure an itchy dog, you may lose time a vet should be spending on the cause. Itching often comes from parasites, allergies or infections.
Taken for what it is, such a bath can still be lovely: a thorough wash in a calm setting, leaving the coat clean and smelling good. People don't leave a spa cured either, but they often leave content.
One more thing applies to every bath. Brush thoroughly first, because a matted coat should never simply be bathed. Afterwards everything has to be rinsed out carefully, since residue can irritate the skin. However healthy the sea may be, a dog with wet mats against its skin gets nothing from it.