Apparently we're much better at water, plants, and fish, than concrete, paint, and sealants.
I know there are texture paints and texture paint kits you can get that can produce convincing stone effects--my wife has used them before. It's possible you could get something like that and then overlay it with a marine-rated epoxy or other sealant. I know there's also people who work colored sands and other media into epoxies to keep them natural and non-shiny, but I really have no experience with it.
Personally, I would look to produce something gritty and natural-looking that blends with your dirt accumulation, because I'm too lazy, er, busy to maintain things like that.