Swim diapers are only for no.2. They don't hold no.1 at all. Most kids PT for that first, so there is less risk to go swimming, even if the child isn't fully trained yet.

I've never heard of this having to wear a swim suit over the swim diaper. What purpose does that serve? Kind of silly on a boy IMHO. Here in Europe, they usually just wear the swim diaper, without any suit.

I know that cruise ships have a different filtering system so no diapered children are allowed but that's kind of a special case.