Swim Diapers are a snug fitting diaper designed to contain messes while baby is in the water.  Most pools now require them and it's a good idea to use them in other waterways to avoid spreading bacteria.
Disposable Swim Diapers become waterlogged quickly and fall apart, clogging pool filters and/or contaminating the environment.
Reuseable Swim Diapers are lightweight and fit snugly around the legs and waist to prevent leakage. They are not intended to be absorbent and so do not have cloth on the inside which would weigh the child down when wet.

Check your child's diaper often and have an extra on hand to change into when you need it.