A Rock dock would be pretty awesome IMHO. Especially if you can get some nice flat rocks for the surface, like slate. If you are planning on having fish, you could incorporate cavities and dwellings for fish that would be really cool to take an underwater camera and explore.
One of the coolest things of swimming in a nice clear pond is being able to snorkel around in a natural aquarium and see a whole new world. My fish are finally getting large enough to be bold, and things have really come alive and interesting.
