Are you looking for this for snorkeling? I'm guessing not for diving, since "floatiness" is not usually a quality divers are looking for, but you don't say.
A thin neoprene rashguard shirt isn't going to provide enough floatiness to matter. If you really want flotation for snorkeling, perhaps look into snorkeling vests, though the paddling gear idea sounds interesting too.
Again, if this is snorkeling, think about sun protection. If you wear a shirt of some sort it still leaves your legs very exposed and you can develop a heck of a sunburn if you're out there awhile. In my experience waterproof sunscreen will wash off no matter what they say. (Or as some say, the most waterproof part of waterproof sunscreen is the bottle.)
You could want to consider a full suit in microprene - good for sun protection. Not all that floaty either, but more than a shirt.