I'd also highly recommend looking into infant swimming classes—the sort where you swim with them. I was reasonably confident about myself in water, but have found them to be a great comfort (as well as _heaps_ of fun). They allow me to feel assured about handling a (wet! slippery!) child in water—and take the two kids swimming on my own. Both of ours started lessons about 5 months old, and are still doing them.

It won't help you 'rescue' them necessarily, but it will give you confidence that they might cope for a minute—which will give you more time.

And they're fun:-)