I suppose the reason we Americans don't often use the term "immersion heater" is that we usually heat our water with gas, not electricity. That's "natural" gas, delivered through the city pipeline, if you live in the city; or propane, delivered by truck to your own tank, if you live in the country. That's how it is in Minnesota, anyway. I suppose it could vary regionally.

Come to think of it, I wonder how natural our "natural gas" is.