Most things on a boat are designed to withstand water-borne grunge pretty well, and vinyl is no exception. Chemically speaking, it has all the right stuff to stay clean. However, the vinyl used in seats is designed more to replicate leather than anything else, and all the little pores and creases that make it soft, comfortable and good-looking provide perfect luxury housing for for dirt, mold and mildew deposited in wet conditions. But these are easy enough to remove, and easier still to prevent in the future.
For most common stains and mold spots, standard bathroom mold and mildew spray should provide excellent results. Many different brands can be found anywhere household cleaners are sold. These cleaners won't hurt the vinyl and shouldn't leave stains when used. Spray directly onto the dirty areas and allow to sit for several minutes. This will ensure the spray soaks into the mold and grime. Wipe with paper towels, then hose down the seat to remove leftover residue. This should remove most of the dirt and stains.
For more stubborn stains, use a small amount of rubbing alcohol. This should only be applied to small areas and shouldn't be used as an overall cleaning solution on vinyl boat seats. Pour rubbing alcohol onto a paper towel or cotton ball and thoroughly work it into the stain. Rub it with gentle pressure to remove the stain completely, then wash the area immediately with hot water. If left on and allowed to fully dry over a long period of time, the rubbing alcohol could severely damage the vinyl. Completely wash and rinse the area.
Cleaning "Eraser" Pads
These "stain eraser" pads are essentially lightly abrasive scrubby pads with cleaner impregnated into them, and they're specifically designed to clean dirt and hard-to-remove grime from surfaces such as vinyl. If other cleaners aren't getting it done, this one is almost tailor-made for the job. You can find them wherever cleaning products are sold. Soak it in water, ring it out and rub the stain, applying heavy pressure. Unlike rubbing alcohol, these cleaning accessories require no additional washing after use.
The market is rife with conditioners, vinyl treatments and waxes designed to keep the polymer material from breaking down, getting hard and collecting grime. A conditioner-cleaner alone will go a long way toward keeping your seats free of nastiness, and easier to clean when dirt eventually does settle in again. Waxes and surface treatments can do one better by filling in those tiny pores and keeping dirt from sticking in the first place. You can find these conditioners and treatments at any local auto parts or boating supply store.