I don't know the most effective way to get that up. I think power washing and wet vac would take some out but also drive the remainder deeper. One way would be to sandblast followed by vacuuming, then do it again and again, but it would take a lot of labor and time if you really wanted it clean.

If it were my garage I'd seal it with some type of epoxy or tough plastic floor coating. If it's the marina's floor I'd talk with the management about their problem and ask them to decide on a solution. If they don't want to do anything, and you want to stay there, you could lay down sheet plastic and tape the edges.