I wear a rash guard over a tankini top all the time, for sun protection. My favorite brand is Athleta. They make some really nice ones. I have a shortsleeved one with a little half zipper at the neck and it's my favorite. It's not skin tight and it hides my belly bulge wonderfully.
I prefer the short sleeve ones in Florida because with the humidity, the long sleeve ones feel too warm out of the water.