Good spots for swimming/pulling towables in Boston Harbor.

[Raider Ronnie]

What Fighingfreak said is good advice.
From Spiniker Island-Bumpkin- into Worlds end would be a decent area for pulling a towable with a bit less boat traffic in that area and giving plenty of shelter from wind!
But beware, they do sail boat races around there (mostly mid week)
What ever you do if you are going to pull a towable around,
Learn the routes the commuter boats travel and stay clear of them!!!

[Van]

If your gonna tube or ski, go outside Nantasket beach in Hull, Nice cool clean water, sandy bottom and little boat traffic.
Personally I wouldn't swim where there isn't a lot of water being exchanged (like inside) that gook is still on the bottom, and all the boats add more. Nantasket faces open ocean and the water is cleaner.

Have fun.