Try Home Depot's acid base toilet bowl cleaner. It smells good, has a thicking agent in it to stick to the bottom of the boat, and cost about five dollars per gallon. 1 1/2 gallons will do a 42 foot boat.

Pressure wash the really nasty stuff off, spray it on with a garden sprayer and let it set about 20 minutes (the hard part for me) then wash it off. On really nasty bottoms you might need two aplications. No scrubbing...and Home Depot is right around the corner to most people.