I think you need to draw the line somewhere, before you think you need a shutoff valve under your shutoff valve....
this is why we have bilge pumps.
I think you are much better off having a really high GPH bilge pump with a backup, or some sort of redundant pump system. This way you are covered in the event there is any breach in the hull.... shaft seal goes....cooling water intake hose breaks..... forget your drain plug..... Adding shutoff valves can only protect in a select few circumstances(where the breach is somewhere in-line above the valve) and will require you to identify the problem (WHICH OF MY 4 VALVES NEEDS TO BE SHUT!?)...where you could just have your bad ass massive bilge system on a floater switch
but, if you want shut off valves, then, go for it... but why not just throw a few spare hose clamps, a screwdriver, and some duct tape in your boat instead... since a leaking hose or torn junction is the only thing having a shutoff valve would help with.... in my eyes at least.)