Never having encountered a water-heater designated a geyser, I've never had to get used to saying it. I think I'd say geezer, but embarrassed that it sounds just like geezer.

My problem is that I know how to pronounce the Icelandic geysir, from which it comes, so I call that a gay-seer. Perhaps other talk about all the hot geezers of Iceland, I don't know.