Uh, marine wind turbines:
http://www.emarineinc.com/categories/Wind-Turbines/Marine-Wind-Turbines/These are common and getting more common. Lots of cruisers use them. An alternator hooked up to a propeller blade, what's not to like?
Marine towed generators/turbines:
http://www.wire-wiz.com/id84.htmlThese provide electrical power from sail (if you're on the ocean) or from current (if you're anchored in a river) or from tides (if you're anchored at the mouth of a bay or tidal lagoon). I don't use these, and In practical terms I don't like them; deploying, recovering, and stowing these towed generators is a pain in the tush (especially if you want to recover it without stopping the boat).
I much prefer running a generator off the prop shaft in all of these situations; it's simple and easy and skips all the deploy/recover/stow hassle, and provides more power, if your boat is set up for it. It's an even bigger win if your boat uses an electric motor; that way you don't even need a generator as a separate machine. OTOH, with an electric motor you can't just buy fuel and then run the motor for hundreds of miles on a windless day the way you can with an internal combustion motor.
But most sailboats aren't built for that; to make it practical you have to be getting enough power to overcome the friction where the prop shaft goes through your stuffing box, and there are bad logistical issues involved in getting belts on or off the prop shaft if one ever breaks because without unmounting the motor or unmounting the prop and shaft itself, you can't get a new belt over the shaft. And in most sailboats you have to crawl around teeny spaces under the lazarette to even get at the part of the prop shaft where you could mount a pulley, so it's a real pain in the ass to install or maintain. Finally you have to mount a prop that's a bit wider (and causes more drag) than would be necessary just to transmit motor power to the water, and that involves modifications to your skeg and rudder to accomodate the larger size prop.
So setting up your boat to easily and conveniently generate electrical power off the prop shaft involves major, expensive alterations, or a customized boat build.
But there are people who specialize in exactly that:
http://www.electricyacht.com/