Available at Boater's World and West Marine.
Buy the 1 lb. tub (vessel as they call it)
Mix up in a garden sprayer per instructions.
Spray down trailer or lift with water.
Spray on Slimy Grimy with sprayer.
- - -It has an Enzyme as its active cleaner which actually eats the stain off.
Spray it down with water again REALLY, REALLY WELL.
Then hand wash, REALLY REALLY WELL.
I do it a couple times a season, and the hull looks brand new each time.