It depends on the water and can vary year to year. During a warm year, the ocean on the South Coast of Mass can stay warm enough to swim in with just a bathing suit up until October. A wetsuit will push that until late October. Now if you are North of Cape Cod or if it is a cold or rainly year, then all bets are off.
However, like cbcbd said once the time change happens then no more weekday kayaking and fall weekends are made for hiking. So with a farmer john wetsuit I could paddle til November, but due to hiking, mtn biking, and Patriots football I am usually done kayaking by early - mid October.
I have friends that use dry suits and paddle year round, but it's not for me. I don't want anything to do with the water in winter.
- darren