I'm just finishing up setting up a new shop - we, too, live on an island. Haven't decided what to heat the shop with, but our house is heated with a propane fired boiler that also functions as hot water heater and supplies radiant heating for the main floor and about 1/3 of the upstairs.
Seems to work well - we've owned it for two years so far. Just getting familiar with the system, but it seems more economical than the forced air furnace we had in ourlast home on the mainland.
All told, we fill our 500 gal tank twice a year (about 750 - 800 gals/yr. total). Our cost for gas - propane (including cooking), electricity and water are about a third of what we're used to...
Our system is a shallow pour over 2x12 truss floor, fully insulated from below with a foot of foilfaced batting. It heats fast - within 20 minutes, and retains good temperature. It is setup to cool in the summer, but we haven't had a need yet.