Yes you can , I do mine regularly . every lift
out to flush with freshwater .
Use the ear muffs , I clamp them on with a speedclamp to keep them tight . I dont plug
the lower hole up but do press my thumb over it for short bursts .
Care has to be taken not to flood the engine by using the hose in the engine bay / feed as too much pressure can force water up the exhaust
resulting in "hydraulicking " . safer to feed the water in where it was designed to go in . I speak from experience !
Also , before winter lay up I put a gallon of anti freeze in a bucket with water and hold it up to cover the leg intakes then run the engines until it has all been drawn through the cooling
system . This coats all the waterways with a/f which has good anti rust properties .