The pool. Outdoor pools fed by the hotel's private spring, at around 37 °C, plus a hot tub using the same water.
The hot tub. Thermal-water hot tub.
The Oyster guide says the water keeps its natural temperature, so you can swim year-round. On HotelsCombined, one guest enjoyed this guests-only thermal pool; another mentions the smell of sulphur.
Guide's tip. Depending on the season, insects can be a nuisance by the pool: bring repellent.