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
scrapping them off
Use a bathroom mold and mildew cleaner, or clorox wipes, then condition your vinyl.
To effectively clean mildew off wood surfaces, mix equal parts water and white vinegar in a spray bottle. Spray the solution onto the affected area and let it sit for a few minutes. Scrub the mildew with a brush or sponge, then rinse with water and dry thoroughly. Repeat if necessary.
with a mild solution of vinagar and water in a spray bottle and a cloth
To effectively clean mildew off your shoes, mix equal parts water and white vinegar in a spray bottle. Spray the affected areas and scrub gently with a soft brush. Let the shoes air dry completely before wearing them again. Repeat the process if necessary.
To effectively clean mildew off a tent, mix equal parts water and white vinegar in a spray bottle. Spray the affected areas and scrub gently with a sponge or brush. Rinse thoroughly with water and allow the tent to dry completely before storing it. Repeat the process if necessary.
To effectively clean mildew off your tent, mix equal parts water and white vinegar in a spray bottle. Spray the affected areas and scrub gently with a brush or sponge. Rinse thoroughly with water and allow the tent to dry completely before storing it. Repeat the process if necessary.