Water, and a very very fine abrasive, followed by an even finer abrasive..

A much quicker way to get 90% of the effect is to paint the dock a nice flat black, and then apply 5-10 coats of shellac (hardens MUCH faster than lacquer), scrape it flat, one more coat, scrape it flat again, and wax it.

Voila.. instant glassy finish, without the dangerous fumes or several months of application time. But it's not quite the same as a true black piano finish.. Pretty Darned Close, though, especially for something as small as a dock.

Cheers