There are excellent diving courses on CC. Ragamuffin Tours is a popular one: www.raggamuffintours.com
Also check out Belize Diving Services http://www.belizedivingservice.com/
I remember there being a non-compete agreement between dive and tour operators in the village so that everyone can benefit from visitors. That might still be in effect and if so fees will be the same. Just check these and out and see how they vibe. Lonely Planet's Belize will have some diving info as well.
Enjoy finding nemo!