What I've seen typically down here is sch. 40 buried up to the seawall then transition to type L copper for the dock runs.
I've also seen the transition usually with a yard hydrant then regular garden hose run along the dock. If the garden hose fails either by "natural" means or if something hits/tears it the fix is quite easy.
Also, depending on your water vs. dock height and how your existing water is set up you may want to consider a backflow preventor.