Boy, can I relate to this! I've been a diver for over 17 years and am open water, advance open water, and rescue diver certified. Needless to say, I love to scuba dive every chance I get. And going on a cruise affords me with some wonderful opportunities to dive in a wide variety of locations. It's a great way to go to several places during the course of a cruise instead of having to fly to each place separately.
While some ships offer good scuba programs, it's almost always better to do your own thing if you're an experienced diver. If you've never dived before and want to learn, do it before you go on a cruise. It's much better, cheaper, and easier doing it before you go than to do it while on your cruise.
If you've never dived before and don't know whether it's something you want to do or not, your local dive shop will almost always have an introductory class where they'll go to a pool, get you fitted in the equipment, show you a few things, and let you try it in a controlled environment. Some people can't handle the equipment, find it too confining, or get claustrophobic, so trying it in a pool is a great way to see if you're fine with it or not.
But if you find it's something you want to try, you will experience a whole new world that is absolutely amazing!