You've probably thought you heard it all about help with stains

[Fishwhisperer 180]

Just a heads-up Lathehand,
Ya gotta be real careful with steelwool around a boat. As you scrub small splinters break off and and embed themselves into the surface. It does not take long before these splinters start oxidising. What you end up with are yellow streaks that follow the path of the water runoff. You will wash them off , yet they will keep reappearing. I use Bronze Wool instead.
As for the Grasshopper poop... Maybe you can do a test spot with Hydrogen Peroxide? I seem to recall it has whitining properties. Good luck!