Figuring out how to get yourself to the island can take a lot of planning. There are many options, from regular ferries to high speed ferries, car ferries and planes. In summer the one and only car ferry books out months in advance for peak days, so think carefully about taking your wheels over. We umm-ed and aahh-ed about it, and in the end left the car behind but didn't regret it. For a stays longer than a few days and for families it would make getting around much easier, but be warned, there can be traffic.
So decision made to go car-free we were left then to pick from the various boat options, as the last minute airfares were on the extra high side. There are high speed ferries available all the way up the coast from Manhattan, depending on your travel dates. In the end we went for the tried and true Steamship Authority, the only year-round and government run ferry, for a 45-minute cruise over to Vineyard Haven. The tickets were 1/10th the cost of the higher-speed options, and we enjoyed the drive to Woods Hole. Parking was easy, we were at a lot a few minutes drive from the ferry itself - keep your eyes out for parking signs along the drive in to town. It was super easy and we were completely happy with our decision. Highly recommended.