The marina in Marathon recently opened a small nice campground. There may be a half dozen sites or so. Most look directly over the water with dock space for an additional fee. There's a new pool next door and just past that a decent bar and restaurant. You can walk to a couple other dining spots and bicycle the upper side of the old 7 mile bridge. For us military retires or DHS employees you can stay cheap at the no thrills sites at Coast Guard station Marathon or Navy Key West.