I saw a thread about this, but I couldn't find where it was. So, I thought I'd post it here since it talks about adult diapers. In fact, I was actually supposed to be looking for something for my story when I got curious about this. It's a good place to start for anything like strollers for adults too. However, the strollers are highly expensive. I usually just use them to link to so my readers get an idea of what the character's 'special needs' stroller looks like compared to baby ones. XD
But anyway, here's a link to swim diaper for those that want to try one or have been looking for one: Swim Briefs | Swim Diapers | e-Special Needs
I think you can also wear them under a swimsuit too.