Why waste money on disposable swim nappies? For the price of a pack of disposable swim nappies you could buy a washable swim nappy that can be used time and time again.
This is a Kushies swim nappy, which says "medium" but I sell these as a birth-to-potty option as they do up with velcro and are securely fastened with a tie, so are extremely adjustable. They are the only swim nappy that I stock as they open up completely for ease of removal if the worst should happen. I never fancied dragging poo down my baby's legs!
The "boy" option is an aqua nappy with fishes. The "girl" option is a red swim nappy with red gingham bindings.