What is the best way to get to dive resorts in Belize. That is, what airport to fly in to and how long travel from there to the better dive resorts. Shuttle? Taxi? Resort recommendations for divers and non-divers/snorkelers teenagers.
Dive resorts are all over the place, so a great deal depends on where you're going. You can fly (or often boat) to the main centres (San Pedro, Caye Caulker, Placencia), but lots of resorts are nowhere near those centres and you'll need boat or sometimes road transport. To get to Hopkins (Hamanasi etc) will take plane and road. To get to any of the atolls and the resorts on them you'll fly to a suitable embarkation point (eg. Dangriga) and then take the resort's own boat - which likely will only go once a week.
Teenagers vary enormously in their demands. One who needs a constant supply of "night life" shouldn't come to Belize at all. One who needs an occasional dose might try San Pedro, so long as the resort is near town. Ones who can engage with the environment and see what there is to see will perhaps be happiest at more remote resorts. On Ambergris Caye I'd point them towards Tranquility Bay, a place where I've seen teenagers strangely content NOT to be "entertained" all day/night.
If you go to a small seaside village/resort, you must expect most of what happens there to be concerned with the sea. So if "non-divers" still like to snorkel, to fish, to go out in boats, to walk along the beach, they'll be happy. If they mainly want non-sea things to do then they've really gone to the wrong place. You CAN take land trips to the mainland from San Pedro, for example, and most people who come here for a week take at least one. But if such trips are your main reason for coming to Belize then you should go and stay on the mainland. There are both cheapo options and some wonderful (and expensive) jungle resorts.
"Night life" in San Pedro (and probably also in other places like Placencia) mainly consists in patronising bars. Drinking & talking, and sometimes listening to live music. Don't expect to find concerts/theatre or anything intellectual except as very rare exceptions. Take lots of books or a Kindle (no 3G service here).