The United States is difficult to sum up. Fortunately, we've only been diving in two places—Hawai'i and Catalina Island—so let's assume that the United States is equal to only those two places.
If you have the time, dive Catalina Island. It's not a great dive: the water is murky and often cold. But it has a beautiful giant kelp (Macrocystis pyrifera) forest, plenty of giant sea bass (Stereolepis gigas), and it's overall a pleasant place to visit. For a good overview of the place, which we wish we'd seen before travelling, see Catalina Island Diving & Underwater Photography Guide.
Hawai'i is a far more interesting place. (And look out for Linus Torvalds!) There's really too much to put into a small review—you can really just hop in a car and shore dive so many beautiful places. We've spent a fair amount of time in Maui and Kona, and would absolutely return to again.