I took the covers off

and what a real ***** to get back on

never do that again now I take them onto the dock and with a brush wash them down with a water aand bleach mix and rinse really good and than leave them out in the sun for very long time maybe even a few days to dry.
Nick
I like Seadaddler's suggestion, because with some variation, it is similar to mine. This is what I have done with good success. Do it on a hot summer day (or hot summer several days) because foam holds a lot of moisture that take a long time to dry out:
> Depending on the actual fabric covering the cushions, take them off;
> Go to Home Depot or your local pool supply store and buy a few gallons of swimming pool chlorine;
> dilute the chlorine with H2O in a plastic 35 gallon garbage can;
> put the cushions in (probably only a two/three will fit at a time);
> let them soak for a while;
> remove and hose down
> do the best you can to wring out the wet;
> do the rinse/wring routine several times;
> let dry in the sun (probably will take a couple of days) to get fully dry.
Do all this on concrete or other surface you don't care about (not on your living room carpet) as the bleach will ... well "bleach". I have a swimming pool so I am able to dump the cushions in the pool to rinse. The process did dual duty ... cleaned the cushions and chlorinated the pool!
If your upholstery fabric is in good shape and is synthetic, the fabric won't shrink. You probably can simply wash them in a washing machine. Use cold water to be safe.
And yes, depending on how tight the fit, putting the upholstery back on the cushions takes time and a lot of patience.