I know Cotuit quite well. Have breakfast at the kettleHo!
Note that you need to enter and exit from Osterville to the west, use the Seapuit river to get to Cotuit Harbor. Read what this guy has to say. Boating_Guide_to_America:State_MA:Place_Cotuit_and _Osterville
All trips in nantucket Sound need to take the East-West current into consideration, and the 3-4 knots in delivers can make a big difference in travel times and in course steered.
If you are new to the area, here's my personal recommendations for over night trips, you can see them all in four days, skip one or two if you don't want to spend so much time on the boat:
1. Edgartown; lots of good bars and resturants and the prettiest Colonial town in new England. A must see. Go here for reservations Edgartown Harbormaster - The Official Site
Nantucket. Same description as Edgartown, just bigger, more expensive, and more crowded. See the beautiful people. Go here for reservations Nantucket Moorings
Vineyard Haven and\Hadley Harbor. VH is my favorite destination, smaller and more quiet than Edgartown, home of the famous Black Dog Tavern, Mad Martha's ice cream, Murdick's Fudge, and Bunch of Grapes Bookstore. See the intellectual beautiful people. Hadley Harbor is just 4 miles away at the Northern tip of Naushon Island, breate-taking place for a quiet lucnh stop. Watch the Forbes families come and go.
Hyannis is a short day trip, tie up behind Baxter's bar all the way into the harbor, have a few drinks while you chat up the crowd, you may get lucky and pick up a few crew for the ride back.