DD had one, I think the brand was Wee Swim from Target, it was a cloth, reusable swim diaper (which I found to be much less leaky than the disposables. I will never use a disposable swim diaper without rubber pants again.) I think it started at 12 or 13 pounds, DD was 13 pounds at 9 months, so I had to look for the tiny ones.
Re: Size one swim diapers??
they make size s, m, l