Maybe active with parking brake.
boats are very annoying to park in the way they won’t move (it is possible but long and unreliable) so it would be good to have a system, anchor or rope, able to tie down a boat for it not to move

I can see about applying "braking" force when boats have "parking brakes" engaged. Would be essentially the anchor you want. Up to you to render the anchor, though. Best get used to the translation variables!

Could we have the translation depend on the sea deepness? Because a default translation will make the anchor allways go the same distance… not good to have a 10 beep anchor when you are in 2 deep water

Smooth brake translations are already a thing. They were added in V15.9.1.

Implemented in 15.10.0.