In California (or at least my part of it) it's pretty common.

I'll echo this. Most HSs that I know of in CA have pools. But I can't find any info online about percentages or breakdowns (urban, rural, suburban, etc). I'd say west-coast states and those that don't freeze have an easier time with pool upkeep and year round usage.

Yeah, I don't have hard information on this either, but I get the impression that it's a much different calculation if you're in a climate where the pool has to be indoors.