As previously said the Rodney Bay area has a multitude of hotel and restaurant choices in all price ranges. the beaches are not as nice in St. Marteen also nudity and topless are prohibited on all beaches as they are all public.
Personally I wouldn't go AI on St. Lucia, too many good restaurants to choose from.
You'll find that in the bars on the Rodney Bay strip it's about a 50-50 mix of locals and tourists.
I wouldn't hesitate to drive back from Pigeon Island or Gros Islet (except for the jump up, cause the drinks and homemade spice rum are very strong) at night, but there is no way I would attempt to drive those mountain roads after dark.
A nice hotel, right on the beach, is the Bay Garden Beach Resort; we stay at the Bay Garden Inn which isn't on the beach but you get to use all the facilities at the other resort and they have a free shuttle bus that runs hourly from 7:00 am till 10:00 pm. there is also the Bay Garden Hotel, also not on the beach, but it can be a little noisy at night as it's bigger than the inn and the bar there has things going on at night (karaoke, managers party for both properties and such)