I do like unpacking new lines, and happen to think these black lizard and green fish print swim nappies that have just arrived from Imse Vimse are rather cool and very smart! 
One question we’re often asked is ‘what nappy should I put inside the swim nappy?’ The answer is nothing.
Quite a sensible question really, as it isn’t immediately apparent what will happen with any wees and poos your baby might do, since your normal nappies are designed to contain and absorb! The purpose of the swim nappy is to contain any poo within the nappy, as this will be nicely full of bacteria and should it float out in the swimming pool will likely result in pool closure for cleaning. Therefore the swim nappy is designed to be close fitting so nothing will escape down the leg or out round the waist. The fabric of the swim nappy is a very close mesh or double layer fabric or in some cases a fabric coated with a lining to contain any poo. This means that whilst the pool water is able to pass through the nappy, as will a wee, any solids are contained so that you can take your child out of the pool without risk of leaving the evidence behind. As urine is essentially sterile – other than if perhaps your baby had a bladder infection, in which case you wouldn’t be swimming anyway - this isn’t going to be a problem with pool hygiene. However if you were to pop your usual nappy inside, this would absorb quite a bit of pool water and could gradually drag your baby down with it’s weight.