There are many nice campgrounds on Cape Cod. Scussett Beach is a state owned campground, as as mentioned, is right on the ocean. the only problem is it is quite a distance from some places on the cape itself. (Provincetown for example) There is another one called Bourne Scenic which is right on the Cape Cod canal, and is quite large. Further down toward the tip of the cape in Eastham is another one called Atlantic Oaks. We've stayed there several years ago, and it was a nice place.
There is another 1/2 way down the cape in Dennisport called Chase Oceangrove, and again, that one is right on the ocean. There are several more, but these will give you some to start with.
Good luck, and have a great time.
Rick/Jan