Swim diapers are required by most public pools these days. If you plan to take your baby in a public pool, it is a good idea to check to see what your local regulations are. In our area, for instance, they require two layers - with one disposable swim diaper and one reusable swim diaper. In most areas they require simply that the swim diaper be snug-fitting enough to contain messes. Swim diapers are not usually absorbent as they are not intended to contain urine (which is sterile), rather they are meant to keep bowel movements contained long enough to get the baby out of the pool and cleaned up. A leaky dirty diaper can mean that pool officials have to drain the pool or close it for several hours while they clean it. How embarassing if your baby's leaky diaper was the cause!
Another reason that swim diapers are not meant to absorb is that a very wet diaper can be a hazard since the weight of the water will pull baby down. So, we carry both non-absorbent and lightly absorbent swim diapers. The absorbent styles will hold a pee or two, but are not absorbent enough to use in place of a diaper at other times.