Swim nappies are a waterproof nappy with no absorbency, or very little absorbency, sewn in. They are designed to keep number twos in the nappy and out of the pool. Most swimming pools require babies to be in swim nappies.
A lot of people ask whether you are meant to add anything to the swim nappy and the answer is no. Just pop it on and go for it. You want to get a snug fit around the waist and legs. If you see any gaping then it isn't going to do a great job. Swim nappies may not contain a big wee.
Some people use one of their nappy covers as a swim nappy. This is fine, just keep in mind that a pocket cover will fill with water and drag, and that chlorinated water may void warranty or wear out your covers faster.
You can get swim nappies in sized or one-size-fits-most and in velcro, snaps or pull-up style.
Do you have any questions about swim nappies?