Camping oceanside is more relaxing in Maine than in Mass or Rhode Island (IMHO), but the swimming is for the thicker blooded. More lake options in Maine or possibly Adirondacks. Lake George is an excellent suggestion. It's got everything, but is very touristy and can be pricey.

Lake swimming in New England can be hit or miss on water temps. Some lakes you can swim all day by mid June, and others are still frigid in mid August.