IHI/danmark paid over $100,000 for my medical care in 1996 - in the U.S. for most of it. There are a few requirements - if you need to be admitted to a hospital you must contact them first so they can authorize the hospital to bill them. Other medical services - you pay out of pocket and submit a claim form and the bills to them for reimbursement. For most of the world their coverage will be 100%. In the US, a whole lot less than that.
It is age-dependent on premiums (the older you are, the higher the premiums), but once you are accepted by them they cannot cancel you.
As a Canadian, you have more resources than a US citizen does - I've got Canadian friends who have had serious accidents while cruising who flew back to Canada for surgery and repair of their injuries.
You should check with your local insurance agent - there is travel insurance available in Canada to cover you while traveling outside of Canada. Not knowing the terms and condition, I recommend that you review them carefully.
If you do not have some sort of comprehensive insurance coverage it is absolutely imperative that you be sure that the yacht owner on which you travel has comprehensive liability insurance to cover any accidents that may occur that injure you. The problem here is that (a) not many owners are going to be willing to produce the insurance policy for your scrutiny, and/or (
they might not have any insurance; and/or © they resent your asking and you've lost your spot [in which case you probably are better off not being on their boat]. Of course, their insurance coverage would only cover you in the case of an accident on board, however, and health insurance would still be needed for illness or accidents when not on board.
I remember a brand-new Australian-built boat came into one of the Manly marinas with the (American) owner at the helm - he backed into a slip and crushed the paid skipper between boat and piling as he was trying to secure the boat. The injured fellow was in terrible shape with a crushed pelvis and other serious injuries. I remember discussion of the fact that he was lucky that he was a Kiwi who could get free medical care in Australia. Who wants to be dependent upon being the "right" nationality to obtain reasonably-priced or free medical care in an emergency?