The Jersey shore is great for swimming and other water sports, and if you can go mid-week you won't hit as much traffic. The drive can be brutal because the beaches are on barrier islands reached by causeways. But there are plenty of choices, from Cape May up to Atlantic City/Ocean City.
NYC is tough in the summer unless you enjoy restaurants, museums, night life and cultural opportunities, which are mostly air conditioned (plus stuff outdoors in the evening at parks and so on). There are zillions of those so lots to choose from. It's hard to suggest options without knowing more about your group.
Speaking from personal experience, I'd say that Boston is fantastic if you want a city on the water, and much easier to get around in because it's small. It's farther from Philly but I don't know where your friends live so maybe it makes sense.
We've also done an inn in a rural area like NH with access to store outlets and walking out on the rocks in a lovely river, if that floats your boat. They always have canoeing, kayaking, and in some places, white water rafting.
If you're a AAA member, why not work with their free travel service?