Summary: Mold and mildew are difficult to eradicate in a marine environment. However, with regular cleaning, you can vastly reduce the occurrence of either.
Mildew and mold thrive on the combination of moisture and dirt. Given that those two elements are plentiful in a marine environment; it can end up as a health hazard. The only way to keep the issue from escalating is regular cleaning.
You will need some form of mildew and stain remover a soft bristled brush, some water, and some clean, dry towels.
Test your mildew remover on a hidden spot on your seat or cushion before you apply it across all the cushions.
Assuming everything went well, spray the stain remover on the moldy areas. Allow the remover to sit for a few minutes. Check the instructions to find the exact amount of time.
Use the soft brush on the worst affected areas or cracks that need some extra effort. Use as a soft a brush as possible to avoid damaging the seat.
Repeat the entire process as many times as it is necessary to remove the mold and stains.
Once everything is gone, rinse the entire surface with clean water. The problem with mold is that unless it is completely removed, it will return very quickly.
Finally, dry the entire surface with a towel, and keep the seats and cushions as dry as possible, especially at night.