*Updated OP with before and after pics* What works best for removing watermark from fiber

Star Brite

star birte

and wear gloves

If those stains are really deep, the only thing you can count on is an acid based toilet bowl cleaner. Ring King is the brand I've used. LINK

BR hardware stores sell (used to sell) this. Been using it for years on fiberglas boats -- I'm too lazy to clean mine after every trip in the Basin.

Before you dismiss this as a joke -- the acid based cleaners work by dissolving//destroying the organic material that comprises the stain. It doesn't harm plastic or gelcoat (as evidenced by the fact that it is sold in plastic bottles).

You'll need to wear rubber gloves. Spray it on or use a sponge to apply. Do NOT get it on your stainless steel trailer fenders (same warning goes for Starbrite) -- fenders will start to rust within 3-4 days. I'm pretty sure Starbrite cleaner is also acid-based, but it has never worked as well (for me) as Ring King.

As was noted earlier -- wax your boat after you clean the stain off.

We sent ours to a detailer after I left it in the water and had started trouble. This is the before and after...

Liquid Bar Keepers Friend

...will scratch a finish.

On & Off Hull and Bottom Cleaner!! Will leave it sparkling white, good stuff!
Mix a 20% solution and apply with brush, wait a min or so then rinse off... DONE

Star Brite or anything else with oxalic acid in it to eat the crud.

Then use a compounding polish to get the fiberglass shiny again.

Then put a good coat of wax on it or two each year.

I have big jug of the 3m micro finishing compound. It's great when I was trying to bring the polish back after a scratch repair.

You need muriatic acid, we used it to clean Mastercrafts after they stayed in the lake for a week or more...It's powerful stuff, we used a 1:10 dilution with water and it would eat the scum right off. It will damage the gel coat if you don't dilute it (the strongest I'd ever go is 1:4) so its better to do multiple sprays (always pre-mixed it ad loaded into spray bottles) with a weaker concentration if you notice any scum coming off at all, and after you finish be sure to wax it. Even if its never been waxed you're going to expose parts of that gel coat that haven't seen weathering in years and the wax will both make it shine and protect it the rest of the season. Can't emphasize how important the wax is for future protection...

Spray it on and don't let it sit any longer than 1 minute without spraying it off. Wear some rubber gloves too because you'll feel it if it hits bare skin..It won't do any damage to you unless your huffing it or shooting it straight into your eyes but it is an acid so use your common sense.

Muriatic is in almost everything labeled "hull cleaner", if you go buy Muriatic from Lowes though you can get a gallon for $8 vs $40 for the watered down version at West Marine. Don't let me scare you off because I guarantee you that nothing will pull scum off better than it will. My $.02

Be sure to clean the boat over gravel, if you do it on concrete, wet the ground first or it'll leave spots where the acid hit it. Wouldn't hurt to wet the trailer too in case any overspray hits it.

source: cleaned boats everyday for 2 summers at a ski school