MIX Iron out in a bucket and brush it on
R & R,
If I leave my Parker 2820 in the water overnight I'll find a brown water mark on my bottom & hulls in the morning. I've also had a friend who'd rarely cleaned his boat and it was a nice shade of dark brown. It took care of both our boats in seconds.
I use a product called On/Off and apply it with a cheap spray bottler or better still and sponge mop and bucket. It's powerful stuff so heed the warning about skin, eye, and breathing. It'll remove years of stains in seconds without scrubbing. It also takes away the wax too. Be sure to keep anything you don't want affected (trim tabs, trailer, concrete, etc washed down with water.)