If US medicare works like it does in Canada, medicare reimburses out-of-country emergency hospital care AT CANADIAN HOSPITAL RATES which are well below what most hospitals south of the border (and elsewhere) would charge. In fact, if you don't carry out-of-country insurance, and you get sick out-of-country, you can face a shockingly high medical bill. So most Canadians are very used to carrying out-of-country health insurance if they travel.
But I recall a case about 10 years ago that hit the media here: an immigrant family travelled to the US for a holiday and their kid broke his neck in some sort of water-related accident. They had no out-of-country insurance and assumed the Canadian health care system would pick up the tab. The kid had to be air-lifted back to Canada after a lengthy hospital stay on a respirator and in a coma. The bill was in the hundreds and hundreds of thousands of dollars and I imagine they are still paying for it.
I guess the thinking is that if you are sufficiently well off to afford to travel, you should also be able to afford the insurance. In addition to DAN, we always tack on extra out-of-country health insurance when we go to Mexico for the winter. Mostly the latter plans DON'T cover diving accidents.