The ICW ‘stash is a common problem.

In this picture, you can see that brown wave-shaped area in the bow.
There are lots of suggested remedies for the ‘stash. Here are a few.
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Oxalic Acid, also known as Wood Bleach. For example, this: http://www.acehardware.com/product/index.jsp?productId=4378333. We did not actually use this. We were intimidated by splashing acid around.
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On and Off. http://www.amazon.com/MaryKate-On-Off-Bottom-Cleaner/dp/B0000AXNNI. We did not use this, either. It's probably very effective, but it's also right scary.
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Lemon Juice. As in cheap old lemon juice from concentrate.
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West Marine Pure Oceans Hull Cleaner. http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=11151&productId=98372&langId=-1
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The Works. http://www.dollartree.com/The-Works-16-oz-Tub-Shower-Cleaner/p6065/index.pro
Yes. The last one really is tub and toilet cleaner.
If I was a real scientist, I would have tried each one on a separate area of the hull so I could compare.

I'm not a real scientist. I never even played one on stage. (I did play King Herod, once.)
Here's what we did instead.
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Wash with plain old boat soap. This is just the basic rinse with water, scrub with soap and the super long brush and rinse with water.
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Scrape the vinyl residue left by the fenders. Wipe down with acetone to clean off anything left over after the scraping.
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Wipe some Pure Oceans Hull Cleaner on the front-most 10' with a sponge. It didn't seem to be working well. Ugh.
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Spray some lemon juice on the next 10'. Take a break for dinner. Come back. The lemon juice seemed to work about as well as the expensive Hull Cleaner. There were real gaps in the brown. Yay!
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Buy 2 liters of lemon juice and a spray bottle fitting. Trim the pickup hose on the spray bottle to fit the lemon juice bottles. Spray a liter of lemon juice on each side. This is a pain in the hand. But, a good, thick layer of lemon juice, left on until it dries seemed to take off a good deal of stain.
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Rinse. Looking better. But not really very good at all. Ugh.
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Pour Pure Oceans Hull Cleaner into the liter bottles. Remove the lemon juice label just to be sure. Spray this down the hull. One liter per side. Sprayed on thick.
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Apply some ScotchBrite pad to the thickest buildup of algae. Abrasives are not a good idea: gelcoat is relatively thin. I think that a putty knife might be better than an abrasive. But I had the ScotchBrite pad handy, so I used it. After using it, I thought about it.
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Wait until the hull cleaner dries.
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Rinse. Looking really good. Yay!
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The white stripe above the bottom shows the most discoloration. For this we wiped on some The Works with a sponge. Then rinsed it off fairly quickly. Not much waiting. And just a little elbow grease on this pass. Mostly a final touch-up to scrub off the most persistent stains.
At this point, it doesn't look new, but it does like right good. There was no small amount of water used in all of that washing and rinsing.

The stain was only about as high as we can reach standing on the ground. So all of this was done without benefit of scaffolding.
It does mean spraying chemicals over your head. Chemicals you don't want in your eyes or hair. (The lemon juice isn't so bad. The Hull Cleaner and The Works, however, require glasses and a hat.)
As near as I can tell, the Pure Oceans Hull Cleaner and Lemon Juice are head-to-head equal in cleaning power.
Since we have a half gallon of Pure Oceans Hull Cleaner, we'll probably use that next time, too. Then we're probably going to switch to The Works. We're told it doesn't require two passes; we should have started with that, not finished with it.