I used these diapers with my son for 5 months last year, in both swimming pools and a fresh water lake. He weighed around 25-28lbs and wore 18mo sized pants for most of that time, so the Large fit perfectly - you Have to have a snug fit in the legs. They are a bit sheer when wet, but the padding in the crotch area make it still ok to use w/o swim trunks over them if you forget them.
Most important - THEY DO HOLD IN POO! Between my husband and I, we had at least 3 big poos over the summer - and the diapers NEVER LEAKED POO. That being said, because you have to peel the diaper down off the kid, it is almost impossible to get the poopy diapers off w/o a bit of a mess. But at least you don't have to worry about causing a health crisis/shutting down the local pool b/c of a poo leak.
They wash easily, and even survive a few accidental runs through the dryer. We took swim lessons 2-3 times a week, and only needed 3 diapers to get us through the summer...