Howdy. I recently acquired a jon that has been sitting in the water for about 10 yrs and there is 10yrs of algae etc growth on the hull at the water line. Does anyone have any advice on what best to use to clean it? Thanks in advance.

Maybe a wide angle spray nozzle on a pressure washer?

Hey Catfish not sure if this idea would work for something that stained but I use Mr Clean magic erasers on my boats water/scum line and they work literally like magic. Maybe spray what you can off then hit it with the eraser. I make the kids do it

If it looks like I envision, and it's been out of the water and dried, you're gonna need some aggressive tools. I would go to Harbor Freight and buy the 5" scotchbrite wheels for a mini grinder. That will attack the algae and not damage the aluminum. I completely de-rusted a steel boat trailer with a couple of those. You'll get real dirty.
Good luck!
IslandJim

From least abrasive to most abrasive:

The order I'd go
1. Power washer

If still some algae on there, put on some rubber gloves and a mask and try the following in this order

2. Simple Green (the stuff you use to clean BBQ grills) with wire brush or Scotch Brite pad
3. TSP (trisodium Phosphate) with wire brush or Scotch Brite pad
4. Aluminum siding algae removers (Wet and Forget Moss, mold, mildew, and algae remover is a popular brand)
5. Toilet Bowl Cleaner