The seats are vinyl and not leather. In the past i have used a patio furniture mildew remover, safe for vinyl. It worked to some degree, maybe removed 50-60%.
Maybe Amazing Roll Off might work, never used it.
I did a search and came up with these ideas. Personally it might be a last resort to use bleach. This first one is quoted from a marine web site. If you are thinking of getting them recovered, maybe try them and let us know if it works.
"I use common bleach straight from the bottle, buy some inexpensive white shop rags soak them with bleach and lay them on the seat for a few hours, don't spill it on carpet,, the bleach will kill and remove the stain won't hurt the vinyl. if there is any exposed stitching don't put the bleach on that,, use some spray wax on the stitching first to protect it from the bleach or just avoid the stitching area."
http://www.kingslandreachmarina.com/...rina/clean.gif