Perhaps it is a trial run of a Project Blueseed type of setup where they plan to test it out in calm waters before moving 12 nautical miles off shore to avoid most US laws.
I would like to state on behalf of
Blueseed that, despite
some rumors, we are not affiliated with or supported by Google, and do not know for certain what the purpose of their four barges is.
However, I've compiled all the information I could find about the barges into this Wikipedia article, which anyone with sourceable information is invited to contribute to:
https://en.wikipedia.org/wiki/Google_barges