Is it even possible to swim whilst supporting the weight of a BMX bike? I imagine a BMX bike would be about 10kg or so. For the Avon Descent they test buoyancy vests by putting them individually into a metal cage, which I think weighs 8kg. They then immerse the whole lot in a big container of water, (a wheelie bin). If it doesn't float then the buoyancy vest isn't allowed. They test all of them, and each has a tag attached to it if it's passed scrutiny.