According to this forum, http://forums.iboats.com/showthread.php?t=295220, Clorox Cleanup is a good solution for black marks on boat seats. The black marks are most likely mildew and will come off easily with Clorox Cleanup without damaging your seats.
simple green or castrol purple power cleaner
scrub it off w soap
you can get good cleaners off amazon
Bit of spit and a rub
Use a bathroom mold and mildew cleaner, or clorox wipes, then condition your vinyl.
scrapping them off
with a mild solution of vinagar and water in a spray bottle and a cloth
That would depend on what material the car seats are made from. Des Perado
use somegoof-off or paint thinner or gasoline
The best way to get the rust off of leather seats is to not let it get there in the first place but if that is not avoidable then you can use any kind of highly rated leather cleaning products such as armor all.
by a wet cloth soaked in salty water