When this question came up recently, suggested were toilet bowl cleaner, bleach, acetone, and soft scrub. my "new" 1978 Sunfish
I'd have to ask, is the deck the original gelcoat or does it have a painted surface? Also, what is the nature of the stains? Brown leaf stains—like oak tree leaves?
My latest boat—which was donated to me after Hurricane Irma—has scattered growths of blue-green lichens. Since it's been painted baby-blue (and I don't have a pressure-washer here) I'll probably try Dutch Cleanser* on a wet sponge. (* $1 at Dollar-Tree stores).
Blood stains have to come out immediately. Ask me how I know...
Use to use Hydrochloric Acid to clean marine growth off of fiberglass hulls. You can
get the same basic thing in toilet cleaner. There is one called "The Works'. That said
it is highly caustic and you need goggles, a face shield and gloves. Use at your own
risk. If you boat has been painted then you can't use this or a pressure washer as it will remove the paint.
If all else fails it might end up easier to remove all the growth and paint with a pressure
washer and repaint. I would recommend staying away from Acetone as much as possible
as this is really nasty stuff and cause nerve damage to exposed skin with repeated exposure
as my father found out. For fiberglass a electric pressure washer has proven quick and
effective for me. I hesitate to recommend a gas pressure washer as it can inadvertently
be set to a pressure high enough to splinter a wood deck. Don't ask because it will never
find it's way into my collection of happy-thought-memories.
There's a product called "Zing"...which is basically muriatic acid...or the family of. Don't drip this stuff wearing tennis shoes. It'll sting! Cleaning waterlines prior to bottom painting has been a task of mine for many years now and I find the Tidy Bowl or similar actually works with about the same results. Let the product do the work. IOW let it sit for a bit and then gently scrub. It's like a "magic" bleach. If a stain doesn't remove with such measures, then more drastic means get called in...like sanding ..to even the point of respraying gelcoat. Gelcoat can be fairly porous and some stains jut embed themselves for life. Kinda like sails... bummer !
My go-to routine on a stained deck/hull is to wash with strong soap (like Wisk laundry detergent) and a Scotch Brite sponge, rinse with lots off water, then flow on laundry bleach and let soak for 20 min. or so, rinse thoroughly and then go after harder to clean stains with CLR. The trick is to let the cleaning products do most of the work and not to scrub too hard to avoid scratching the glcoat. Wear gloves and eye protection.