Massachusetts has an absolute TON of waterfront areas, including places where dogs can splash about on the shores. In reality, there are few places better equipped for keeping you and your dog cool. Just make sure to check out leashing and time restrictions, as most beaches are different.
It might be somewhat short and sweet, but taking a dog-friendly Water Taxi can be a super fun way to get in between places to hike and swim.
While their fresh menu is a bit more limited, Polka Dog does offer several healthy frozen yogurt treats for your dog, plus an escape from the sun inside the store, which is littered with ultra-healthy and all-natural dog treats you can take home and save for later.