I own a company that cleans slipways (we have contracts with Princess Yachts PLC, Mayflower Marina and Plymouth City Council) www.cleancoastservices.co.uk
How we clean is as follows ... Firstly we pick up and rubbish/weed by hand. Secondly we pressurewash with powerful pressurewashers. Thirdly we rinse thouroghly. Fourthly we spray (using normal backpack garden sprayers) Chlorine, the same stuff you use in swimming pools. This is very efective on the slipway and does no harm at all to surrounding areas (we have tested this greatly!) The chlorine comes in crystel form, mix 3 cup fulls to a bucket of water and stir until completly dissolved then simply spray a coating onto the slipway. You will see a diference in about 5 mins. The chlorine will extend the need to pressurewash the slipway again up to 4 weeks and that's in the summer when the weed grows the fastest.