Even if you don't otherwise use cloth diapers, get a cloth sim diaper. The disposable swim diapers just get sogged out and horribly uncomfortable for baby, and start to sag so they can't actually catch any poop, which is their only purpose (the pee comes out in the water). The cloth ones fit snugger and are much better at containment. Nobody wants to swim in poop soup.
As far as flotation devices, you and your baby will figure out what you're comfortable with. My son was very timid of the water at first and putting a (properly fitting) life jacket on him helped him build the confidence to do more. Now he's fearless in the water, and I'll let him go without the life jacket only if there is another adult there with me, so someone's attention can always be on him. With DD, we started with an inflatable swim ring, which she liked at first, but then always wanted to be out of it. She is absolutely fearless (at 16 months) and was at the point where she was just jumping in without caring whether I was watching or not, so we put the arm swimmies on her, which keep me sane. When she is big enough that a life jacket will properly fit her, we will get her one, too.
SAHM to Declan (12/12/06) and Blythe (2/9/09)