An observation,
While visiting a friends retirement home site I made careful observations of a dock that he had built there. It is a very large E shaped dock with two bays for boats to park. It also had hoists mounted to a beam in the roof to lift the boats out of the water. This area has lots of rock and the posts for the docks were just set on top of the rock, no footer, not anchored to the bottom. The roof in combination with the deck gave the dock it's stability. It also made a nice place to store a few fishing poles.
As a sidenote, metal roofs also require less framing than shingle roofs and go on much faster.
I'm not a big fan of the type of metal roofing that screws/nails through the exposed ridges. Not a big deal for a dock but a house is a different story. In my opinion it is risky to rely on a rubber type washer to be water tight over 20+ years.
