There are at least a dozen companies you can buy water from and have it delivered to a giant storage tank made by at least 5 different companies. Or you could use a number of companies to dig a well for you.
Many people don't have access to municipal water and when I didn't, this is what I did.
The main, monopolistic solution of municipal water certainly is. The point is that options can still exist in a monopoly. Google still has a monopoly despite the existence competitors.
Firstly it's not structurally practical to store giant tanks of water on private property. So the comparison makes no practical sense anyway.
But more, there is a significant market for private water in other properties. It's a non-trivial market which serves a significant proportion of the US population.
There is no significant market for private/non-Google search. Not only is DDG a footnote, but Google's dominance forces businesses to take steps to optimise their Google rank - or face penalties by losing access to potential customers.
Google can also remove businesses who aren't even customers from its rankings on a whim, with no redress.
When I lived in China, I didn't have the luxury of having access to Google, so I used Bing instead. It was actually quite reasonable for most of my needs, I'm not sure why more people don't give it a go. Of course, now that I live in the states again, I use Google, but most of my searching is on YouTube anyways (which was something else I didn't have access to in China).
I'm pretty sure those same apartment buildings wouldn't let you set up the large scale server farm required for you to set up your own search engine alternative to Google either.
Many people don't have access to municipal water and when I didn't, this is what I did.