It's been 35 years since I spent my summers on the Cape but I'm pretty sure the water is still warm and comfortable for swimming on the bay side. Cape Cod Bay is shallow and protected and much warmer than the ocean side. At low tide, it is enjoyable to walk way out and wade through tidal pools looking at the stranded sealife. The downside, is that the waves are way smaller if you're into surfing or just jumping into the big rollers.

I would guess you could find a beach rental if you look. Try Orleans and Brewster. Better hurry, though. I've no idea of the prices but would guess it will be more expensive than the Outer Banks. And don't do it if you don't like crowds. Cape Cod in the summer was Hell for me and I haven't been back in 20 years. It can only be worse.

If you do it, have a clam roll for me.