It is a structure built on four barges (last time I saw it; there may be more in the final design).  I believe that it's a data center built to be powered by wave action and cooled by seawater.  But, more importantly, it's a boat with a 'zero-point' wave energy engine.

To explain: there is a lot of energy going under a boat all the time in the form of waves.  We have spent much of our effort in marine engineering learning to withstand this energy, the way a bridge withstands buffetting wind.  But it is also possible to take advantage of this energy, first by capturing wave power to operate something onboard (in this case computers) and second, if we are selective about what wave energy we capture with vertical movements of the barges and what wave energy we allow to push us around, we can both derive energy from it (like a windmill) and use it to move boats around in a purposeful steered way (like sails). 

I anticipate that when it is built they will keep it in the bay for tests for several weeks to months, and then move it out into the 'potato patch' (a section of notoriously rough waves right outside San Francisco Bay) for harsher testing and possible deployment.

If I were to speculate about *why* Google is building such a thing, I would note that right now the appropriate database to 'autodrive' cars is starting to exist but the appropriate database to 'autodrive' boats is still out there bound up in thousands and thousands of charts, of various sources and ages, from different companies, and that many of the available charts are just plain wrong.  So I would guess that this is a prototype of the 'Google maps' vehicle for bays, estuaries, lakes, harbors and coastlines, as distinct from the 'Google maps' vehicle for roads. 

If you're going to make it, I'll host it 

Are these the same size as the ones at the Dollar Tree?