Look here: https://www.google.com/about/datacenters/efficiency/internal... . PUE globally is 1.06, compared to Yahoo/Cap Gemini Lockports 1.08.
(Disclaimer: I work in (another) Google datacentre.)
I love how you use things like seawater in Finland and non-potable water elsewhere, but don't claim this DC in SC is even remotely environmentally friendly when to save on electricity and keep your PUE low, you are using millions of gallons of drinking water per day.
But you're right, regarding to water usage we don't publish numbers (nor does anyone else afaik, I haven't found any WUE number for e.g. Amazon, Yahoo, Facebook...) so it's difficult to compare this to others.
I am however confident that a lot of things have been considered and this was the best option, we have imho a good track record trying to do the best for the environment, a lot of attention is being paid to this internally. E.g. everything running on green electricity, which is not the cheapest.
Also, like you said, these metrics only take into account the efficiency of the DC but fail to measure their ecological footprint.
On the other hand, Yahoo is constantly shat on (sometimes validly), yet they solved this years ago. Their Lockport, NY data center does not need air conditioning because of its design and the cool local weather. It uses 50 percent less electricity and 99 percent less water compared with traditional data centers.