Is Medicare of any use for dive accidents?
Medicare doesn't apply outside the US, so if you have a dive accident or get ill in a foreign country then you are on your own. If you need hyperbaric chamber treatment and/or assistance returning to the US - it could be very expensive.
We hope to retire within a year and so I went to a seminar on Medicare and Medicare Supplemental and Gap policies just last night, and it is incredibly complicated and it's not cheap. They weren't trying to sell us anything at this (park district) seminar, but it was clear that if you don't have comprehensive health coverage you could spend most of your life savings on health care and not leave much to enjoy your retirement, or to provide for your spouse.
There are many different Medicare supplemental plans to address gaps in Medicare coverage. Gaps, ceilings, exclusions, out-of-pocket expenses, premiums, maximums, and co-pays that vary for inpatient, outpatient, medications, long-term, and hearing/vision/dental care. These supplemental plans are administered by private insurance companies and the coverage and rules may vary significantly by policy and by state and/or county here inside the US.
For example, if you plan to travel or even live in different US states (e.g., spend your summers in the midwest and winters in the south) then you have to make sure to select supplemental/gap plans that will cover you in various US locations.
So if you get ill outside the US, I don't think that you can expect much assistance. I'm sure that there are some supplemental plans that include foreign travel, but they might have higher premiums and they may not specifically cover diving-related illnesses and injuries.