look on ebay for the cotton bottoms ones. we had some for ages but gave them away ffp a while back. they're just a wrap i think but u can use them in the pool, we did with no issues. and they often have some funky patterns on too.x
We started taking nathan at 4 months and used a imse vimse medium one with a medium happy swim nappy over the top.i'm not sure how big the smalls are but nathan was 8lb 6 at birth so mediums might fit.
I started ds swimming at 6 weeks - started off with a 'happy nappy' that the place made us use, very very quickly moved onto a kooshies swim nappy. They're quite good because they're like a wrap so easy to get off when wet.