We have some of the iplay brand swim diapers/trunks, and they DO absorb - just a little. We were at a splash area one day and DS was suited up to get wet but didn't really. I was holding him when he peed and could tell, though it was *mostly* absorbed by the diaper, because it was not already wet. Still, I won't put them on until we're ready to get in the water lest there be a puddle on the floor.
I would also really hesitate to use anything as absorbent as a prefold, because of the water being absorbed and weighing baby down. I realize that infants aren't trying to swim and move around, but I don't think having an extra 5lbs or so on them is necessarily safe, kwim? As posted earlier, just a plain cover of almost any sort (with good elastic) will hold in poos, and that's all you're really looking to contain, anyway. We've used empty pockets, as well. Do avoid velcro, though, or it'll almost certainly stick to your suit!
