In almost all cases, bolts used in marine environments are either hot-dip-galvanized or stainless steel. Zinc plated fasteners offer very little in the way of corrosion protection.
Hot-dip galvanizing is a process of applying a protective zinc coating by dipping product in bath of molten zinc, and the zinc bonds with the steel. Zinc plating (or electroplating) is a process where zinc is applied by using a current of electricity. This coating is much thinner than hot dip galvanizing and in general is not specified in outdoor environments, especially marine environments.
It should also be noted that galvanized nuts are tapped oversize to accommodate for the added thickness from the galvanizing process on the bolts. A zinc plated nut would not fit properly on a galvanized bolt.