Check the price of seaborne, cape air, air sunshine and inter caribbean air. Sometimes it is cheap and sometimes not.

I like to use wiki for the airport I am going to and see who flies there and from where.
https://en.wikipedia.org/wiki/Terran...tional_Airport

STT usually cost a bit more than SJU.

You could also do a private boat/sail and hit some great beaches and snorkel spots on the way over from stt.