This is one time I would personally stick to a ship's tour. Ferries in the BVI run on Caribbean time. A 9 0clock ferry may leave at 915 if a local is running late , same goes for your return. Ferry, Tortola to Virgin Gorda, is only a few dollars. If you go to
bviwelcome.com there are links to ferry schedules.
You can walk to the Baths, it is about 2 miles uphill. I suggest a cab, about $3.00.
I will be going in May, I've been to the Baths and will be taking a ship's excursion.