“Make them on the cheap and let them fall into the ocean after the mission.”
I absolutely agree on making them cheap, but the drones I envision are not airborne, but rather very fast boats just large enough to support the gatling gun, a couple small missiles, and whatever the latest underwater anti-torpedo ‘missile’ is.
They developed something like that a while back for harbor defense. I believe it was one of those ideas to protect harbors after 9/11.