Been There
Is this an aluminum boat? There is a product called Toon Bright that I used to remove two years worth of that stuff off a pontoon boat. It didn't hurt the paint on my trailer. I pressure washed it first. They have one formula for aluminum and one for fiberglass.
I just did a major clean up on this boat. I shared all I leraned in a thread titled "Green Stuff" just a couple of weeks ago. I had lots of advice from others that may help you too.
DP
I am a heterosexual male. 2 Chronicles 7:14
"If my people who are called by My name will humble themselves and pray and seek My face and turn from their wicked ways, then I will hear from Heaven and will forgive their sin and heal their land."