rgano, (and everyone else for that matter)
Best solution for a boat sandal, is Keen footwear. They make a nonskid, nonmarking sandal with toe protection. Even a flip flop.
Also, not all black soled shoes are marking, in fact, these days most aren't, though it can be hard to tell.
Problem with most boat shoes, sperry, sebago, etc. is that while they are great for your boat they are lousy for your feet and back. No support whatsoever unless you put some sort of orthotic in them.
Here is the best place to buy Keen btw.