Depending on how many are in your party, it may be worthwhile to charter a flight from STT directly to EIS. (Island Birds and FLY BVI are ones I have used.) It is a short hop, much easier, more dependable, and quicker than the ferry route. EIS is very close to Trellis Bay, which is where you will take the ferry to Scrub. As others have stated, there are also smaller airlines that fly from STT to EIS that would be worth looking into.