How picky are they going to be about the note? I would probably just make myself some fancy looking letterhead on the computer at work and type my own damn note. I don't think they are really going to call and verify it.

If you have a regular doctor you have a good, long-standing relationship with, s/he may be willing to write one for you just based on the information you provide if you explain the reason why you need it. I've found doctors are often willing to write doctor's notes for...less substantiated reasons...when it doesn't have to do with more legally complicated issues like work excuses or disability. I'm not saying they would lie (an ethical doctor wouldn't do that), but they may not require testing for something like bringing beer on a cruise ship when you could simply describe your symptoms and they could diagnose from that.

Maybe you could ask a naturopath if your regular doctor refuses?

You should be able to get a note from your doctor either MD or ND. Yes, an ND is a real doctor. Yes, their notes are accepted.

It is worth the trouble to get the letter now in case you are ever hospitalized.

There's always Dr. K!!

If my current MD were of a mind to respond to your request with skepticism, I would use that response as an occasion to find a new MD. I did said MD shopping 5 years ago, and am delighted to have a truly fantastic functional/integrative doc for inspired conversations and first-rate treatment. (Jeepers, my family's primary care physician when I was a lad, chain smoked and was often seen stumbling drunk at the golf course. I HATED everything to do with visits to that creep's office.)