Degreaser, a brush, and some good, old fashioned elbow grease.
Depending on what material your shoes are made out of, that foaming shoe cleaner might be effective.
Or, you could just be like me and simply not care how ratty your shoes get.
Try a bit of rubbing alcohol on a cloth. You'll need to re-polish the shoe later.
Finger nail polish remover
Depending on what it is. If it is just rubber scuffs, just use your palm and rub it. It may hurt a little because of friction, but it is likely to peel off. What the marks are, basically, is just melted rubber from the shoe. So friction, eg:rubbing your palm into it, will likely melt it again and lift it. Another way is polishing it, to try cut it out.
Use a Magic Eraser cleaning pad. That's the name of it: Magic Eraser It works especially well on white shoes.
There is an eraser for suede. It will rough up the nape and the water marks will disappear.
spit in your car
You take it to a black smith's and get another shoe!
you could use baby oil or go to a pharmasist and they could recomend something.
Black shoes would be suiteable for a white suit
You get a bulldog with permanent black marks that are hard to scrub off?
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.
Black shoes are obviously black in colour.