I swam in the ocean in Hawaii for hours and stayed warm (I'm prone to chill) with no sunburn at all.  Very comfortable material.  I bought a medium (I'm usually small) and it was a little looser than I would have chosen with a try-on so I say it's true to size.

I wear this to stay warm at the pool with my kids. It definately helps, especially when I get splashed. It's very cute but I wish I had bought the large. I normally wear a small so I thought a medium would work but it's pretty tight. Perfect for a surfer I suppose but as a mom at the pool I would like it a little roomier!