Jan,
We are also going to Belize later this year, and I have been searching for info online. I dont know if your particular ship will offer a tour there, what Ive seen online looks like passengers are getting themselves there using independent companies. From what I understand, once you tender into Belize City, you can then grab water taxi's to the different cayes (opperated by local companies). I can put some of the site links here that I looked at and found helpful.
<http://www.belizenet.com/belizefacts1.html>
<http://www.coralbreezelimited.com/>
<http://www.virtualtourist.com/vt/9c3/>
<http://www.gocayecaulker.com/>
We are planning to use Coral Breeze and take their tour to Caye Caulker... we are wanting a relaxing, laid-back day and that is the impression I get from reading about that particular caye. However, the one you mentioned does seem to be more for those who want to snorkel and swim. Good luck, and let me know if you come across anything else interesting.