Many of the premiere boating clubs on the South Shore [1] are private clubs, but there are also marinas like Onset Bay (RFD #3, Green St., Buzzards Bay, 508/295-0338, www.onsetbay.com [2]), which repairs and stores boats, and Borden Light Marina (1 Ferry St., Fall River, 508/678-7547, www.bordenlight.com [3]), which provides fueling and docking services.
Links:
[1] http://www.moon.com/destinations/new-england/massachusetts/eastern-massachusetts/south-boston/the-south-shore
[2] http://www.onsetbay.com
[3] http://www.bordenlight.com