Is your boat on land or in a marina? If yes then the best solution is to stop it happening in the first place, and install a portable dehum on board. You can pick them up for a few hundred $$$. The improvement is very noticeable & instant. No more mold, rust, damp, bad smells....

With my boat, the cushions were dirty and oily so I took off the covers to wash and then found the foam needed cleaning too. So I soaked the covers in detergent and nappy soaker, the foam I washed in warm soapy water by hand.Some of the covers were stitched onto the so I washed them as is. The loose covers can be washed in the washing machine and dry quickly. The foam takes forever to dry so after the third day in the sun I snuck them into the washing machine and spun dried each one, they still took several days to dry properly. It's better to get the covers off if you can because the water in the foam stains them a bit. If the zips are seized try using silicone spray or even try undoing under water. Once they are clean give all the covers a double coat of scotchguard spray you'll probably need at least three cans.