Funny, half the world wants to go to New England in Sept/Oct for the fall color; you want to leave it.
September and October are both still hurricane season, so while the weather in Caribbean/Gulf Coast/Bahamas/Bermuda/Mexico could be great, it's a definite risk.
If you can be happy visiting gorgeous red rock country with excellent hiking, then you might consider Sedona, AZ. Fly to Phoenix and it's a less than 2 hour drive north from there. Another possibility with beach (tho I don't think snorkeling) hiking, probably deep-sea fishing possible, and nearby wine country is Santa Barbara, CA, which is a 4 to 5 hour flight from Boston, I'd imagine. You could fly to LAX nonstop and drive north; maybe there's direct service to Santa Barbara from Boston, but I doubt it.