I'm not aware of any such tax breaks, and I don't think there would be any motivation for it around here because there are not any harbors on the entire coast that lack for boats.
Finding a mooring can be very hard in Massachusetts. A friend of mine a few years ago bought a boat, and then had to wait a year to bring it up here (from the tropics were he bought it) because he was unable to find a mooring. So aside from the difficulty of finding a mooring, here are a few of my favorite spots.
Salem Sound is of course quite nice with good day and weekend sailing destinations, and there are a number of boat clubs and public mooring fields around Salem. Occasionally I've seen a Salem mooring for sale on Craigslist, so might keep an eye on that.
Rockport and Gloucester are very nice, but they can be quite a drive unless you live on Cape Ann.
Newburyport is a very pretty spot, but the daysailing destinations are somewhat limited (basically Isle of Shoals).
Portsmouth is nice too, if a bit industrial in some areas, but again the daysailing destinations are limited (not that I don't love IOS, but it might get a bit dull).
I love Portland, and if you'd consider heading that far north you'll be rewarded with much better cruising grounds, and probably an easier time finding a mooring. There are two public mooring fields that I'm aware of, off Portland Head, and off the East End. Both have nice locations, although the Portland Head field has better scenery. If you find yourself striking out for mooring availability further south, Portland could be worth considering. You can cruise a lifetime in that area and never get bored. Also Portland is an easy days-sail to the southwest end of Penobscot Bay.
