In reply to nickb:
I know nothing about water, or boats, but releasing a prussik under load is not always easy - and can't be guaranteed in an emergency especially if you don't know the load before you set the system up. I would suggest considering keeping the prussik and release elements separate. The document I pointed to shows a classic prussik can be very, very strong. So, how about combining this with a munter-mule-overhand on the locker. This also allows for a more controlled release.
The other classic, non-controlled, prussik release mechanism is a knife. Be it a real one or a 1mm cutting cord.
As you probably know, any line under large forces suddenly released is a fast moving dangerous object. But again, I know nothing of water and boats.