Firstly, are you needing just to get stains/grime off the outside fabric? If so, previous suggestions seem great ... Not much different than the approach for cleaning a home sofa. Can't see why any carpet/upholstery cleaning firm can't do it at your home.
I notice that you are in WI. So if DIY, take the cushions from your boat to your well heated home before doing. Or wait until spring with several warm and low humidity days in the forecast. Immediately after surface scrubbing with upholstery cleaner, suck moisture away with a wet-dry vac. Do it again with only water to remove the cleaner residue. Dry in the sun.
If mold/mildew has invaded into the foam, then the foam itself can be improved by soaking in a mixture of water and swimming pool chlorine in a plastic garbage can. Drip dry. Wring out. I've driven my car tires over my foam to accomplish this. (DAYOR ... Do At Your Own Risk on this!). Then let dry for several days in hot sun. Has worked for me.
But if as indicated by a previous poster, the fabric and foam on your boat are fused together, and you need to clean deeper than just the surface fabric, then perhaps a flood recovery type of firm would be who you should consult. I suspect that if you have the cushions at home, they can do it pretty easily.