My off shore experience has been that ALL of the injuries in rough water have been below decks. "One for the boat etc" doesn't work when there is nothing to hold onto. I had two injuries on my last off shore trip: one head, one broken rib, both of which can be life threatening. After that I gave considerable thought as to how to make it safer below decks. On my boat the only practical solution is some sort of interior "jack line" set up from the companionway forward. Not as good as adding overhead grab rails, but I think it might keep reduce a "sLam" to a "slip & slide". They will need to be well tensioned and of course I only have a couple of places that will take the strain; the companionway rails, and the compression post, but that takes care of the gauntlet area anyway.