Hi ktwillie01. I'll do my best to answer the questions that I can, but there are much more knowledgeable people here who can help you with the diving information. If you decide to certify, you can do the first part of the course locally and that will save you a lot of time and time studying instead of being in the water once you are here. I'm a lapsed diver, who is now quite satisfied snorkeling, but that doesn't mean everyone would be. That said, the snorkeling here is quite spectacular, but there some things and experiences that you can only have as a diver, so it really does depend on what you want. Certainly learning to dive and diving in Belize are absolutely amazing experiences.
I've not been to the Blue Hole myself, but understand that it's a pretty long trip if you're just going to snorkel. I have had one or two people report that they enjoyed the experience thoroughly though.
You can truly spend as much or as little as you like on eating and drinking in San Pedro Town. If you stick with local drinks and local eateries, you could budget for $100usd for two of you, but on holiday sticking to a low budget can be a bummer. Think about how much you'd spend to eat out and drink out say, in Florida, and estimate from there. Always better to over than under budget.
You can hire boats with a captain to take you fishing, snorkeling and exploring. You can't do this legally without a licensed captain/guide as far as I'm aware.
Hope you both have a wonderful time. X'tan Ha is a gorgeous resort!
AmbergrisCaye.com 
