Manta Rays prefer open waters so chances are slim. I grew up in Belize and only saw mantas twice (and only jumps), once on the way to a two week camping stay at Half Moon Caye and Northern Two Cayes and more recently, a few years ago fishing off of Glovers Reef.
Now spotted eagle rays are fairly common and there is nothing quite like snorkeling eye to eye with one. The areas that I have seen the most (btw...this is one of my favorite fish to observe) is next to Carrie Bow Caye down south and if you are ever at South Water Caye, they come into the lagoon just about every morning to feed. This past summer I "visited" with one just about every day that is was at SWC and the reason I knew it was the same eagle ray is that it had three barbs on its tail.