If I were going to keep a boat in that area I might also consider Marion. It's not as protected as Onset, but there's more open water outside (Buzzards Bay is wider there)... It's got more sailboats than any other minor harbor I can remember. Maybe as many as Newport.

Each harbor has a different flavor. We've been getting to know the harbors in Southern New England, and visited most of the harbors there (several times) but I haven't been to Whareham yet.