"I want hot water, and you?"
Wow took me a while to realize this. Don't speed-read characters, i guess!
Living in China three years, and I've still never heard it called 热水. Only 开水.
I also hear 开水 more frequently than 热水, though I do hear plenty of both. I guess it's a "soda" versus "pop" thing; just depends on what people in your area grew up with.