The 'then': Go down and by some nice fender covers that will protect your hull and the fenders too.

If you want to clean them: i) Buy a "fender" cleaner (or "power cord" cleaner) at a Marine Store. It's a petroleum based product that should wipe away a lot of grime that a cleanser won't touch; or,

ii) Hinckley's charter fleet maintanence crew chief suggested an amazing product which you SHOULD use carefully only on rubber, SS, or plastic (NOT ON PAINTED SURFACES). It's called Butyl Strike (by Sprayon). They use the Lemon variety. It cleaned diesel residue off of our MOM 9 polypropolene case that NOTHING would touch. They use it on inflatables too.