Been on a Carnival cruise to the Caribbeans a few years back, and my only advise is go for the smaller ships, if you can afford it that is, as they tend to be pricey. If you are going for cruise on a budget look for deals with Royal Caribbeans or Celebrity, both lines have a good reputation regarding their "floatels".
I also suggest the following site to help in your decision making
http://www.cruisepeople.com
Thanks, that's actually pretty helpful. Budget actually isn't much of an issue but it's something I never thought about, so I'll look into it. But the people whom I'm doing this for don't think smaller ships are safe or sanitary but some of the reports they've been reading about them vs. the "floatels" seem to be changing their minds.