Don't let your baby's diapers keep them away from the water!
If your baby isn't potty trained yet, swimming diapers are essential if you want to allow them to enjoy any public body of water. Regular diapers won't do the job. Trust me! They will turn into a waterlogged anchor around your baby as soon as they touch the water. They'll become so saggy up to the point of falling off your child. Therefore: useless!
Swimming diapers are made to contain solids and withstand water. They are not meant to absorb. And yes, to answer your question, pee will pass through the water.
Note: Swim diapers aren’t designed for newborns whose stools tend to be liquidy (until they start eating solid food). Lacking absorption power, they just can’t contain newborn poops. The same goes for children with diarrhea.