We have just returned from Marriott Vacation Club Frenchmans Cove -- had 2 relaxing weeks there.
Yes -- definitely need a rental car if you wish to do your own exploring and beach visits. The roads are relatively basic and often steep & hilly. We had a Jeep Wrangler from Hertz which was ideal. I Don't recommend a conventional car such as a Taurus. They use US left hand drive cars but drive on the left so that is something of a challenge for US people [we are from UK] The Marriott properties are a 10 minute drive from the town -- but this can be very congested when cruise ships arrive. Best beach Magens Bay, 20-30 minutes from Marriott.
Check out : http://www.vinow.com/
Have a great time. Fiona & Murray