Good boat shampoo
I use about 2 tap-loads of product in a 10 Lt bucket of water. Plenty of foam. I apply it with a scrubber brush suitable for deck washing. After spreading it, I let it rest for a minute or two before scrubbing. Once the scrubbing is done, I remove it with plenty of water. This product removes most dirt from the deck and gelcoat surfaces. You may need a hard scrubber for the tougher corners and anti-slipping surfaces. For grease or harder stains I use a small undiluted bit directly on the scrubber.