I'm a Baby swim specialist teacher, and without a doubt the best swim nappy is the Popolino (Motherease) one (They are the same, but Popolino are made in Europe under licence to ME, and they are cheaper). These are front fastening, so if they get filled they are easy to remove. They are lightweight and pretty, with lots of lovely prints. They are very adjustable - large size will fit from around 6 months through to a good 2 year old. They wear well, and as they popper open, there is no velcro to catch on baby's tummy, or wear out in the chlorine. If you want a stockist please PM me!
Whatever you do, don't buy pull up swim pants - if they are filled, they are a nightmare to remove from a wet baby, especially as the chances are you will be wet as well!!