Chilly is right on regarding February. You definitely want to book in advance and not just show up. Just pay attention to cancellation penalties. Ambergris Caye is not one of those "cancel up to 6pm the day before" places. In terms of diving, I would not pre-pay for a dive package unless you felt you had to. If you stay "in town," there are several dive shops up and down the beach. Everyone on this forum has his/her own preference. Each shop has its own "vibe." Pick one. If you like it, stay there. If you don't, walk down the beach to the next one. In terms of depth, if the DM says he's going down beyond 80 feet, just tell him/her that you will only be going down to 80 feet. The DM will be fine with that if the DM can see you. You will likely find other people doing the same thing.
On a non-diving note, check TripAdvisor for restaurant recommendations. Yes, I know some people don't like that site but I have found it very helpful for food. You can filter out the noise if you read a larger sampling of reviews. Ambergris Caye has incredibly cheap street food that tastes great as well as high end places. Again, there's something for every budget on that island.