Sized swim nappies are designed for babies in a certain weight range, so are great for getting a perfect and trim fit. Most brands come in two or three sizes.
Like in regular MCN's, one-size-fits-most (OSFM) swim nappies are perfect for those of us who love a bargain. You only have to buy one swim nappy and it will last from birth (or close to it) through to toilet training. Most OSFM swim nappies have snaps on the rise to adjust the size of the nappy to small, medium and large settings. If your baby is little or has skinny pins, then you may not be able get get a snug enough fit with a OSFM swim nappy in the early days.
Pull-up nappies are preferred by some swimming pools and this option is generally sized. This style of swim nappy has elasticised leg cuffs to get a really snug fit and become leak-proof.
Other swim nappies are fastened around the waist with velcro or snaps like regular MCN’s. This option is great when you have to deal with a dirty swim nappy as it comes off a little easier. This can work the other way too, if you have a little nudist in the making - toddlers can take this style of nappy off easier than the pull-up style swim nappies.
The main thing to remember, no matter what swim nappy you choose to go for, is that you need to get a really good seal around the legs and waist to avoid leakage. You don’t want to be the person responsible for clearing out a pool!
ps. the swim nappy in the picture is the very cute Seedling Baby Paddlepants in Explorer print.