There are various types of plywood available but in all instances plywood to be used in the marine environment should be constructed of hardwood sheets, and ideally be of British Standards 1088 (BS1088). This standard outlines that the wood should be free of voids and bonded with WBP (water boil proof) adhesive, which wont break down in the presence of moisture.
Plywood is a unique material in that it offers the hard wearing properties of timber, with high levels of strength and corrosion resistance, in flat, nearly homogenous, sheets. This makes the material quick and easy to work with - In the past a timber deck was laid down in numerous longitudinal planks, all of which have their own tendency to wrack, turn and warp, but thanks to plywood, a single uniform sheet can be bonded over the deck beams and sealed with deck paint or epoxy resin, etc. As Well as reducing time and skill necessary for the job, the areas where moisture can permeate through the deck is greatly reduced.
Like everything else in the marine environment, plywood it is not infallible. Plywood members mostly rot out of sight where moisture is present, behind bulkhead tabbing for example. It is understandable then why many 'project boats' suffer from rotten bulkheads, as they have been left unloved for decades, and are often full of water.