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.
One last note - don't change into the swim diaper until you are ready to get in the pool. Like all swim diapers, they are designed to hold in the poo, not urine. So if he wets himself on the way to the pool wearing the swim diaper, everything is going to get soaked.