Hot Water Heaters?

In today's Real Estate section of the New York Times, there's an article by Jay Romano called "How to choose a hot water heater." It details everything you'd ever want to know about water heaters. I found it kind of interesting.

Besides the headline, "How to choose a hot water heater," hot water heaters are mentioned throughout the article.

I feel (not too seriously, though) that I need to point out something that maybe William Safire in his column On Language should write about. If your water is already hot, why do you need to heat it? You don't heat hot water, you heat cold water to make it hot. So shouldn't these things be called cold water heaters, or just water heaters?

I wonder where the expression comes from about someone getting into "hot water."