Just like a regular diaper (or pull-up). You can also use them outside the water, but they don't hold as much liquid. Some pools require you to also use plastic training pants over the diaper to protect the pool from leakage of anything solid. I've never seen any brand other than Little Swimmers, so I don't know about that.
Our community pool sells the plastic pants, but you can also get them online (Gerber brand) and we used them later on over underwear for potty training, too. But if your pool doesn't, I wouldn't worry too much about it - I don't think my kids ever pooped while in the water. Just don't feed them right before swimming.