In past years Target has had some nice life jackets -- of course, if you're crusing right away, they don't have them at this time of year. They were nicer, I thought, than the ones at the Disney resort pools and are available in nice girl-colors (probably boy colors too, but I don't have boys so I don't really look for bright blues and reds). Plus I think it's good for the child to become accustomed to having the same one all the time.
Here's a hint from our swim instructor at the YMCA: tie a swim noodle (you know what I mean -- they're everywhere in the summer) around your child's waist. It'll keep her upright in the water. Unfortunately, this gives my youngest daughter WAY TOO MUCH confidence in herself!
As for swim diapers, the washable panty-type are MUCH better than the disposables. They're very inexpensive at
Walmart or Target, and they're also available through mail-order catalogs. Neither of my daughters ever pooped in the pool anyway, so it didn't seem to be a big issue. You can wash it out in the sink just like your own swimsuit.
If you insist upon the disposables, here's a hint: did you know they're washable? Yep. I accidentally washed one once (didn't dry it, of course). They won't last forever, but you should get a few washes out of them. They do take about three days to dry, so this is probably more of an at-home tip.