Is it safe to run earmuffs on an inboard/outboard boat motor. If so,, for how long?

The water hose can supply plenty water for cooling, but don't rev it up much over 1000 anyway -- the underwater exhaust ordinarily puts a load on the motor, which isn't there when it's not submerged. The unloaded engine is at risk for "thermal runaway" in which bits of heated carbon in the cylinders begin to ignite the incoming air/fuel charge prematurely, causing the engine to race. You won't be able to turn it off, since at that point it doesn't need the spark plugs. With nothing to hold it back, it can overrev way beyond its design specs. You can try using the choke to flood it out...if you're lucky you'll be able to get it to stop before it tears itself to pieces.