We had only one full day in Martha's Vineyard and it rained all day. When we first woke up we considered cancelling (and forfeiting the money). We did not ... and we are so glad that we did not. Our tour guide, Rich M, was wonderful, with an extensive knowledge of the Island that he was glad to share. Fortunately, there were some breaks in the rain, including when we stopped at Aquinnah Cliffs, one of the tour highlights. The last town visited was Edgartown, and Rich let us exit the tour van there because our hotel was there. We had quickly learned that taxis and Uber and Lyft are all in short supply, throughout Martha's Vineyard and getting a ride from Oak Bluffs (tour start/end point) to Edgartown (where many hotels are located) is both a challenge and a hefty expense.
One final note -- there are several companies offering very similar tours ... all take 3 hours, all have similar names, and all leave at 10:30 am. This can be very confusing, and people often show up at the wrong bus.