Alphonse Graniel ("L'il Alphonse") is a fantastic guide on the reef. His Shark Ray Alley/HolChan Reserve is a fun 1/2 day tour. I booked through Monkey Business Travel, located at Banana Beach Resort the evening before. One day I found I had unexpected free time so booked Alphonse again (!) right before lunch for his afternoon excursion.
You're gonna love AC!