D.W.
They are used instead of the diaper but don't put them on until you are ready for the water because they don't hold pee at all. They are really made to contain poo.
I bought a reusable swim diaper from my cloth diaper store. Paid $10 and because of the way my daughter is built I never had to buy another one! It's always better to have a back up but I never got around to buying one since I had a few leftover swim diapers (disposable) from her brother that I'd kept around.