Why do we need two”swim diapers for swimming lessons? For the first reason, loose poop in the pool will close it down for 6 to 8 hours. It instantly clears the pool for sure. So to keep everyone safe and healthy put on a disposable diaper first then put them over. This protects your reusable diaper from a mess. The second reason deposable diapers legs and waistbands aren’t snug enough to keep body fluids in. The reusable legs and waistband are tight enough to protect the water and your child. I recommend having disposable diapers on delivery. Side note regular diapers under a reusable diaper don’t work because the diaper fills up and then you have a five-pound weight around your kid’s waist also not very safe. plus when the diaper comes off do it away from the water. I have seen on more than one occasion diapers and swimsuits being removed on deck, it isn’t acceptable unless you’re at your own pool, then you can do what you want.