There are many other investments in cancer research and space travel being made by individual Googlers.
However, it is perhaps more to the point that both Google Search and Google Academic are crucial research tools for medical research and for aerospace engineering, as for many other fields.
Both directly and through its employees, Google is also funding substantial applied research in crucial areas such as renewable energy production, genomics, public health, and ground transportation.
I don't know what you mean by "disastrous public plumbing in the US". My experience with US municipal water supplies and sewers during the 29 years that I lived there was that they were inexpensive, reliable, safe, and efficient. My experience with water supplies (and sometimes sewers) in other countries has been much worse.
A personal note: I haven't ever tried to work for Google because of, essentially, ethical concerns: I am in danger of having to live in a world where a large fraction of our communications are intermediated by a single company, and I think Google's cultish secrecy is corrosive to the values of Silicon Valley. However, I think Google should only be dinged for its real faults, not imagined ones.
Google can afford to pay taxes in the countries in which it operates. That it does not, while obviously legally defensible, reflects poorly on the other work it does.
Public plumbing: the US infrastructure is badly out of date, and unable to meet most projected demands for expansion. The US Geological survey projects that 1.7 trillion gallons of water are lost through leaks in the system each year -- about $2.6bn in value. http://www.epa.gov/awi/distributionsys.html
$2.6 billion per year is about US$9 per person. Another point of comparison, if you're worried about conservation of fresh water, is that the Mississippi River discharges 110 trillion gallons of fresh water per year into the Gulf of Mexico, where it turns brackish. So the plumbing doesn't sound like a disastrous problem to me, particularly compared with problems like 1% of the adult US population being imprisoned, companies being raided for selling chemistry kits to kids, a decaying passenger train system that commonly delivers passengers several hours late, and government bailouts to pay the bonuses of the bankers who wrecked the economy.
That's fine, but there are plenty of other people who disagree with the assertion. If the money is in the hands of the government, at least those other voices get a say in how it is spent.
The opportunity cost for anything you spend $3bn on is very high. A government could choose to spend it on cancer research or space ships. Google can really only spend it on a very narrow range of products -- and can't combine it with other serious numbers required to do the really heavy lifting, like that required to fix the disastrous public plumbing in the US.
If Google paying its taxes would mean that it couldn't do all the stuff it currently is, that's one thing. But they've got $33bn sitting in the bank. They can more than afford to pay their taxes and continue creating Wave and AdWords.