We personally like Alphonse Graniel best of the guides we've gone with for 1/2 day trips. He's got a real love of Belize and its marine life and he enjoys sharing his knowledge with wisitors. We've also gone on several night snorkels with him and had a great time.
Because Alphonse doesn't have an office, like Seaduced, Hustler or Amigos, some resorts are hesitant to book with him. So if your resort balks, you might have to push.
Try a search of previous posts - you'll find a lot of feedback about Alphonse and other guides.