After a quick glance at the items already listed it looks like most things are covered. I would add to the list spare fuses, fuel filters etc...Things that make the boat run that could go bad on a trip out. I now keep a basic tool kit, with filter wrenches, fuses, pliers, a few screw drivers etc.. in a dry box in the console. I have already used the fuses a few times. Spare bulbs for running lights should also be in there which come to think of it I'm not sure I put back the last time after changing them.
Personally, I am not sure I would want to do the spark plug, prop thing on the water. If the seas are calm enough I would just drop anchor and call Sea Tow/Boat US. If the seas are too rough to go on the hook odds are you won't be able to do that maintenance on the water either and a call to the tow boat service is still in order. Spark plugs would probably be a different story if I had a two stroke but the likelyhood of that being the issue in a newer 4 stroke just doesn't seem to warrant their inclusion in the tool kit.