Hi there.
I just got back from Nosy Be (and the rest of Madagascar). So there's my advice:
If this is the first time in Africa, and you only have two weeks, stay on the islands and don't venture further on the mainland. Travelling there is tedious, and although the national parks are great (like Ankarana), you'd miss out alot just getting there.
The diving in great. Like being put in a tank. Alot of diving schools claim to speak english, but they do so only marginally. Here's one who speaks very well english: Alain Blondeau of "Madaplouf" in Bemoko. The diving school is great, very competent, with nearly new equipment, and very customer friendly indeed (snacks and tea/water after the dive, pick up and delivery from your hotel in the vicinity). I can highly recommend them. Check out their website for more information.
Hanging out on the beach in Madagascar in general is not a good idea next to where people live: it's basically the public toilet. Clean beaches can only be found in Andilana up North, and those stretches that belong to hotels.
The islands around Nosy Be are well worth a visit. spend at least four or five days on Nosy Komba (e.g. chez Juliette, Bungalows right by the seas side for a mere 8U$). There is a diving school as well. Nosy Komba is known for its habituated lemurs (they jump on your shoulder for a piece of Banana), but there's so much more: a snorckeling trip around the island on a pirogue, a hiking trip up the mountain with Yvonne (you can do it yourself, but it's more interesting with her as a guide, that is if you speak French...), and a visit to the neighbouring village which constructs pirogues of all sizes. There's no cars on Nosy Komba, so it's really relaxing.
Nosy Sakatia has undisturbed and for a change pristine beaches, and the snorkeling around the sacred mountain is superbe. There's frequently giant green turtles to observe. On Nosy Sakatia there are four possibilities to stay overnight, so a choice for all kind of budgets, but it can also be visited from Nosy Be as there is a boat service to and from shanty beach.
Nosy Iranja for a day or two (they offer camping overnight on the beach there) to see a true tropic paradise island. There's a modest fishing village and an old light house (built by the constructors of the Eiffel tour) on the big island and a hotel on the small island.
Nosy Tanikely is great for snorkeling and diving as well. You can't stay there overnight, but almost all dive school offer day trips. I actually combined it with my travel itinerary: I took a pirogue (80'000ar = approx. 55U$ all incl., divided by two) from Nosy Komba to Nosy Tanikely, spent the day snorkeling and eating there, and continued to Nosy Be in the afternoon. The pirogue returned to Nosy Komba afterwards with the other passenger.
April is the beginning of the diving season, and the rainy season should be over by then. It'll rain though, but only for a moment and things dry quickly afterwards. Taking a teenager along is no problem, as long as the kid knows to entertain himself. Apart from diving, there's horseback riding on Nosy Be, you can rent a quad (with or without guide), there's the Lokombe reserve to visit and there's after all a culture to explore. Geckos and chameleons are usually found around the hotels (plenty of insects to feed on and no predators but the occasional cat) Otherwise there's not much entertainmaint suitable for a kid at night, so bring your lab top and some movies and games.
Nosy Be is the most touristy place in Madagascar. Which means it's a bit spoilt, overprized in comparison with the mainland and there's more package tourists than individual travellers found. But it also means it's got a far better infrastructure than the mainland, some people do speak english and actually developped a sense of what visitors would like to see and do. And "touristy" means you'll see a white man occasionally (unless you stick to your hotel or to the "hot spots" hellville and the beaches Southwest of it, so you actually have a choice). Prizes are still cheap for Westeners and people are genuinely friendly, helpful, and gentle. The food is better than on the mainland, and the souvenir shops have the greatest selection.
If you have any more questions, feel free to pn me.
have fun and enjoy
Berit