---> BLEACH (not on wooden boats, OK for fiberglass or plastic) Please test this in a spot first!
---> Use rubber gloves & a sponge with a Scrubby side. (The larger green sponges avail. at most grocery stores)
Have a fan running in a garage or better yet do this outside. Wipe straight bleach (or half water/half bleach) on boat bottom with the sponge side, wait about a minute, then scrub with the scrubby side of the sponge & rinse well with your hose.