Caribbean vacation advice?

Planning a work/vacation trip this summer: About 10 days on St.
Maarten (with some work for my SO each day), then about 10 days
elsewhere.

Interests: some novice-level diving (me), maybe a resort-dive course
(SO),
protected snorkelling (with our 5-year old child), birding or other
cute animal viewing, amateur astronomy (probably with ST80, maybe just
with Canon 15 x 50 IS binos), naval history, good food, scenery.

To be avoided, if possible: golfers, cruise ship effluent (human and
otherwise), shopping as usually practiced by cruise ship effluent.

Based on a quick skim of a few sources, we're currently leaning toward
St. Kitts or St. Vincent for "elsewhere." Any suggestions of which of
these, or other islands nearby, fulfills these criteria and are easily
accessed from St. Maarten would be appreciated. Prefer moderate price
level ($100-150 per night).

Mike