Rules of the water says smaller ship yields right of way to bigger ship. Sounds like you screwed up if your yacht got hit by a bigger ship. Of course that applies when the vessels are not tied up. If a big ship his a docked boat, that's an entirely different scenario
And of course there's the old tale:
Americans: Please divert your course 15 degrees to the North to avoid a collision.
Canadians: Recommend you divert YOUR course 15 degrees to the South to avoid a collision.
Americans: This is the Captain of a US Navy ship. I say again, divert YOUR course.
Canadians: No. I say again, you divert YOUR course.
Americans: This is the aircraft carrier USS Lincoln, the second largest ship in the United States' Atlantic fleet. We are accompanied by three destroyers, three cruisers and numerous support vessels. I demand that YOU change your course 15 degrees north, that's one five degrees north, or countermeasures will be undertaken to ensure the safety of this ship.
Canadians: This is a lighthouse. Your call
https://en.wikipedia.org/wiki/Lighthouse_and_naval_vessel_ur...
There is a lovely bit of complication with it! Not saying to correct you, just because i think it is a lovely bit of trivia even though it has nothing to do with space debris.
Both sailing and power driven vessels need to give ways to (among other things) “vessel restricted in her ability to maneuver”. And an aircraft carrier launching or recovering aircraft is considered to be restricted in her ability to maneuver (quite rightfully so, it is hard enough to land on them without the ship swerving left and right).
So that means that a mighty aircraft carrier needs to (at least according to the regulations) dodge tiny sailing ships, but once they start launching or recovering aircraft it is the responsibility of the sailing ship to avoid them.
Source: Rule 18 of the ColRegs (The International Regulations for Preventing Collisions at Sea 1972)
Where do Methalox-powered vehicles lie in this hierarchy?
And if the Starship is not under power yet falling and using the flaperons for control, is that considered "under sail" for purposes of right of way?
For funsies i would call it a “vessel not under command”. At least after the breakup that feels to fit.
But for real, i think the simple answer is that debris falling from space is outside of the scope of the ColRegs. Simply speaking they come too fast so you can’t maneuver your vessel out of their way, and unless you are a warship you don’t have the tools to even know where exactly they will hit. If you try to run you might even put yourself in their path. After all from the most unlucky position they would be just bright stationary spots on the sky getting ever so slightly bigger. Until they start to get bigger faster and faster. (Constant bearing and decreasing range being the hallmarks of an impending colision.)
There is a whole hierarchy of right of way on the water, but a better rule of thumb is that the less maneuverable boat generally has priority.
https://www.whoi.edu/wp-content/uploads/2020/04/Abbreviated-...