In season, Cape Air flies from STT to EIS a couple days a week. Last minute fares can be very reasonable. Our friends were planning to take the ferry, then discovered that flying would cost about $100/each way, which about equalled taxi, ferry, taxi, and saved many hours travel time.

The other airlines mentioned above fly the SJU route, but I'm not aware of them flying to STT.

If there are enough people in your party, chartering can also be a good option.