From the pictures I've seen, it looks like a giant data center. Google published a patent on just such a data center in 2009, which uses sea water for cooling and wave action for power.
Ever since the Web was spun, there has been an assumption that the Internet could live outside of the law. That has come to seem naive in recent years, but a floating data barge brings the dream alive in a new way.There is a more prosaic theory -- that this is a store for Google Glass, turning the mystery into a marketing stunt. This story comes from Allen Martin of KPIX in San Francisco, a CBS (CBS) affiliate, and the idea is that the completed "store" would be towed to Fort Mason, and then opened to the public. But Martin's story debunks itself. He notes that there is no permit on file to put a barge at Fort Mason and that such a move would have to be temporary. I have another theory, based on past Google practice. For years now, Google has been putting small data centers into shipping containers for co-location near phone company switches. The data centers carry the results of most common queries. Google can "cache" these results and serve them without the query going back to its headquarters. That means faster service and lower costs. The facility in San Francisco Bay is obviously much larger. If it does contain shipping containers, with computers and networking inside the shipping containers, there's enough capacity to handle all the data Google usually has. A second such barge is reportedly under construction in Portland, Maine. The Portland Press-Herald has a picture showing what look like 48 units lashed together, the ends of six covered by wood. To bring such a barge across an ocean would seem to be an impossible task. But if the unit is built and tested here and then taken apart and loaded onto a cargo ship like the Maersk Alabama, subject of the new Tom Hanks movie Captain Phillips, its transport becomes practical.