That was my first cruise 12 years ago, although then it stopped in Jamaica instead of Belize. As a cruising & diving virgin, I did an "Intro to Scuba" excursion on Grand Cayman, and it was an incredible experience. If you are a certified diver you have MANY more options. If you wish to take a cruise sponsered scuba excursion, sign up early (first day of cruise) or risk being shut out.

The Caymen islands are world renowned for their diving...I highly recomend you do it!