MdP is the Meca of the local turism for us. Every argentine takes Mdp in his heart! Jajaja! It's truth!! Mdp recieves 3 million people every summer.
The temperature of the water is not warm, definitely. But you can enjoy it without problems. In summer can rise up tu 25 C'. I've took this information from here:
The weather is special... you have days of a terrible hot, and days that make you put a sweater on, specially at night...
I was there last weekend, and the temperature was 30 C.
Thanks for your comments!