We went last October over half term with the kids and the sea was easily warm enough to swim in, it was absolutely lush. Could stay in for ages. Swam for literally hours each day in the sea.
The y heated hotel pool was cold, but the kids still swam in it. However there was a limit to how long they managed due to it benign cool.
I don’t think I would want to do a water park day I. I heated pools then. However might want to check opening months on them anyway.