I haven't had to try this yet, but the Care Card that came with my boat recommends this for seats with mildew:
(Medium-soft bristle brush)
Step 1: Mix one (1) tablespoon of ammonia, 1/4 cup of hydrogen peroxide, 3/4 cup of water, rinse (fresh water), then dry.
Step 2: Warm Soapy water, rinse, dry.
Step 3: Protect with marine-type vinyl seat finish/protectant.
Try this link:
http://www.marinespecialtiesgroup.com/msg.html . Click on the Products square, new page opens, then click on Cleaning Recommendations, then click on a stain for tips.