Here's a good place to learn about the various types of fuel cell and how they work. General background, not marine-specific.
A look at the likelihood and conditions under which hydrogen-driven fuel cells will work on any boat, let alone your boat.
Ballard is one of the first companies to offer PEM fuel cell products and electric drives for mass markets. Their site explains how the systems work and outlines the expected improvements in performance expected over the next five years.
This company is looking for boat owners who want to participate in demonstration programs, such as the X/V-1 systems which uses a watermaker, electrolysis unit, hydrogen storage, hydrogen-consuming stack, and electric motor.
Steve Cooper chucked his Rawson 30's diesel engine and replaced it with an electric propulsion system. He manages without a generator, so range and speed are limited, but motoring is very smooth and quiet.