My nieces and nephews were always wearing regular disposable diapers when they played around in the water. But if you're getting your child swimming lessons, I think it would probably be easier for them to learn wearing a pair of swim shorts. I'd think disposable diapers would float and make swimming awkward and cloth diapers would get heavy and pull the child down into the water. Do they still make the plastic pants that fit over cloth diapers? If so, perhaps put a pair of those over the swim trunks to keep the nasties out of the pool.