Via slashdot, Google's patent application for:

A system includes a floating platform-mounted computer data center comprising a plurality of computing units, a sea-based electrical generator in electrical connection with the plurality of computing units, and one or more sea-water cooling units for providing cooling to the plurality of computing units.

Cooling is one of the greatest power needs of a data center, so using cool ocean water would help make up for the expensive power.  From the description, it sounds like Google is imagining these as near shore, deployed temporarily in situations like natural disasters where on-shore infrastructure has suffered.  The patent includes a long list of ways of getting power, including Pelamis generators, tidal systems, wave systems, wind turbines, and cables to shore.

Some of the commenters point out the difficulties with computers on the ocean, like the effects of salt spray on electronic components, and on a platform's constant motion on hard drives.  The former will be easy to protect on a seastead, but for the latter, perhaps seasteads will want to have gimballed server rooms?  Or by then, maybe we can just use solid state drives.