The caldeira is really beautiful to look at, a piece of paradise. We also went swimming, I'd say the water is around 28-30 degrees. The place is only 5 minutes from the road, so the whole visit will take you less than an hour.
The area is currently being developed into a genuine tourist attraction. Iread in the newspaper that construction is due to be completed by end of August 2013. Afterwards there's a visit center and the entrance will cost 2 Euros, which will totally be worth it. They estimate 350 visitors per day.