I would like to see ships lockable, that is a really great request. How about this:
Any ship parked in Main waters should be able to be destroyed, whether locked or not. If a kingdom ship is parked on it's own island (i.e. black sail on Pirates, or red sail on Samurai) then they should be invincible. However, this should be limited to parked boats touching a dock or shore, so abandoned boats out in the water can be sunk. Any boat moving at any time should be fair game.
Alan