Well, unless you're a glutton for punishment you probably wouldn't want to spend two weeks in the Belize district. Belize City is um, rough around the edges, and you can see and do what there is to see and do in that district in a couple of days.
But the rest of the country's one big tourist happy pot.
Since you're going for two weeks, you might want to divide your vacation between a caye or cayes and then Cayo, so you would dive one week and hang out in the jungle the next. Or vice-versa.
Or maybe head south to Placencia, where I'll be going next summer for two weeks (but it will be my third time to Belize, not my first), and split your time more equally between diving and running around the mainland.
Imho, it's hard to go wrong in Belize.
As you probably know, the "signature" dive there is the Blue Hole. Some hate it, some love it. There are threads on this forum that can help you decide which camp you're in. If you decide that's a dive you'll want to do, then that will probably influence where you stay for at least some of your trip.
A great travel book is
Insight Guides Belize. Three hundred and fifty pages chock full of info, maps and color photographs. Lots of great ideas on where to stay, what to do and what to see.