I let you know something right now, they are fake termas. There isnt the slightest smell of sulfer in the water and the water isn't hot. Not to mention that entre rios is not a beautiful provincia. Me and my family went and we were not impressed. It's just like the argentinians to make a place with warm water and then slap the termas sign on them to attract tourists.
Where did you go?
I go to Pirapolis in Uruguay by the way. Hotel seems nice, I can do a small trip to Colonia(I have been there already,my girl not) and for 950 peso for 2 for 4 nights, I don´t think a lot can go wrong.
Ive been there several times and have stayed at the downtown hostel, in a colonial type house. Its called Sophies hostel and has 2 private rooms in addition to dorms. I always go to the public pools which is right beside the river with a nice view. the entrance fee is low. Its a nice little town and apparently is booked solid in summer.
I was told by locals that the Colon public spa was once a natural spa and 10 years or so ago the town developed it into regular concrete swimming pools, some outside some inside. Its not as warm as Salto, but its a nice setting and unless its freezing cold outside, youll find yourself warm enough. There isnt a strong sulpher smell. I drank the Salto water directly out of their tap and dont recall a strong sulpher taste either.