I agree with ne_traveling_man. Several that on or really close to the water that you might want to check out are the Wentworth by the Sea in New Castle, NH; the Marriott Long Wharf in Boston and the Newport Marriott in Newport, RI. The Newport Marriott recently underwent a major restoration and has one of the new M Club lounges.
All of them will provide you with a great stay. I've stayed in all of them (many more times at the Wentworth) and am sure that you will be happy at any one of them unless you are interested only in properties actually on the beach.
I agree with nhtraveler; I have stayed at these 3 locations as well - Wentworth by the Sea was in early October so it was offseason rates and allowed for a nice trip with my wife's grandparents!!
I have not been to the Newport Marriott since it has been refurbished but should consider a trip there next spring!