Boating shoes are a mystical thing.
Folks are content to see white soles because they know white won't mark (well, it may mark but on a typical white deck that's invisible) but at the same time, black Vibram hiking boots won't mark either. The problem is, they scare the heck out of the owner who doesn't know Vibram isn't just black rubber. And I've also seen 'street dirt' mark decks, even with white soles.
You'll find many gym coaches say "no street shoes" for the same reason, they don't want schumtz tracked in on the gym shoes.
In a sailing school, do the best you can. Once you get past the question of non-marking, there's the more difficult question of traction. Classic Topsiders, etc. are useless on a wet deck while some sea boots with "octopus" pods on the bottom are great--but wear quickly and pick up gravel off the boat.
You do the best you can. If it is nice and warm, "AquaSox" or similar white-soled flimsies work very nicely. Cheap, good grip, no marks. Black neoprene divers' booties also work well in colder weather but neither has any foot support. Good deck shoes? Priceless. if you find something you like, but two extra par and stach them away, they'll be off the market next year.
