It may not fall from the sky, but it can sure sink quickly. That makes it dangerous. You can make someone else sink quickly. As such, you are a liability to others. Also, most planes can, in theory when there is an emergency, find a place to land. That may or may not save them. When at sea, in an emergency, you cannot simply find a safe port.
I do not find comparing cars to boats apples to apples at all. Cars do not sink. Cars do not get beyond a safe port. When your car gets a hole in it, you can get out and walk. In general, you cannot swim back from a boat.
The world is dangerous. You can't legislate it all away.