Well, for me at the swimming pool I always had the kids in swimmer nappies but at the beach I wasn't so strict. If we they were regular "poo-ers" and I knew they had already done them that day then I would let them be naked on the beach (a bit of wee in the sand can't hurt) and put little trunks on to go in the water.

I wouldn't bother with swimmer nappy AND swim suit unless it's hot in which case the all-in-one one's are good to protect their little skin.

HTH! Have fun swimming - and don't forget the 5 seperate bags full of everything else that goes with the beach lol...towels, sunscreen, change of clothes, floaties, buckets, spades, rakes etc etc!!!