By the time you get to a vessel large enough to have any of these systems integrated they'll very likely have means at their disposal to seek immediate aid. That and systems such as this are often redundant and almost always have means to be operated manually. So, please, spare us the 'easily lead to death' fear mongering. That's just plain wrong.

Two is one and one is none. You'd be utterly foolish to go to sea without redundant navigation and communication equipment. A hacker might take out your navigation instruments, but so could a simple short circuit. Saltwater is brutal on electrical systems, so only a fool operates on the assumption that their primary nav system is absolutely reliable.

Even on a tiny inshore yacht, you'd want a handheld GPS, a handheld VHF radio and a stash of AA batteries in case your main electrical system fails. On a small coastal yacht, that stuff would be in your survival bag next to the liferaft. A large offshore yacht should have two fully independent nav stations on completely separate electrical systems. If all that fails, you should still have an EPIRB and a canister of flares; good bluewater sailors can happily complete a passage on sextant and compass.

Contingency planning is at the heart of seamanship. If you're placing total faith in your instruments, then you have bigger problems than the insecurity of the NMEA protocol.

I'm not trying to be alarmist, but seas are dangerous.