You're familiar with contrails, the tracks left by airplanes as they move across the sky. Those are made when hot water vapor from the exhaust of jet engines hits cold, high-altitude air and condenses into ice.
Under the right conditions, big ships can leave a very similar trail, but it's caused by a slightly different mechanism. Incomplete combustion of fossil fuels means little particles of dust—aerosols—come out in the exhaust. Water molecules are attracted to these aerosols. As water builds up around a piece of dust, you get a cloud. Yes, it's basically cloud-seeding.
This photo of oceanic "contrails" over the Pacific is one of NASA's Images of the Day today, but it's not the first time they've featured this kind of photo. This is a cool phenomenon and they've posted photos of it in 2002, 2005, and 2009.
Thanks Philip Bump!