If they are the canvas type, lightly scrub with mild detergent solution (no bleach). Rinse, dry and spray with waterprofing stuff. If they are plastic, do the same as above at first, then if necessary use a mild abrasive cleaner containing bleach. Rinse thoroughly.
Look, can you stop masquerading as me! I'm Nick and I claim seniority. If you keep this up you'll get Haydn and Byron and Mike Finch and all these other guys confused and they are at an age where more confusion is not good for them!
As far as cleaning your canvas is concerned, get it spread out on the pontoon, use the Staybrite canvas cleaner on it, leave to soak for a bit and then attack with a domestic sized pressure washer (not the big garage/marina variety or you will blow a hole in it). Works well. I advise wearing wellies/waterproofs and giving the boat next to you a good wash down afterwards as all the green mould, etc will wash all over it.
Most sail/covermakers will do a professional clean. Cost is £1 per lb weight typically, so £50 ish for the cockpit covers on a mid sized flybridge boat. I use Lymington Sail and Tent. The clean takes about a week so best to do it mid season when the weather's good
I believe there is a strange liquid which is made by that be-kilted people who live north of Hadrian's wall. Apparently it is made by boiling roasted, germinated barley, then allowing this sort of porridge to ferment. They then evaporate this mess, which gives off a powerful antiseptic and algicide. Now this would not be so bad, but they then ruin it by soaking it in oak for umpteen years. However it does not detract from the algicidal properties, and can be used very effectively, by soaking canvas in this, by now amber coloured liquid. The canvas will never go green again, and actually smell quite nice. Unfortunately, there is one hitch. One of the be-kilted ones- namely one Gordon Brown- seems to have hit upon an easy way of making money, by putting rather a lot of tax on it. So much so, its use as a general purpose algicide has declined, and is now actually drunk by the people of the region - presumably to give them inner cleanliness- strange lot up there!