>Communication and navigation are more difficult because GPS and satellite comms don't work underwater.

That's a really good point, how do you get a signal to a submerged unmanned submarine under all that conductive saltwater? Military submarines use VLF radio waves that can penetrate saltwater fairly deep, but those long wavelengths require enormous aerials and powerful transmitters that are one thing if you're the navy of a nuclear power, quite another if you're a shipping line looking to make a profit.