Over/Under-Qualified Considering Engineers Job

What you are describing reads more like attitude problems rather than qualification problems.

One brings the other.

I don't agree with that statement. Job over-qualification might bring out an existing bad attitude, but I don't believe it creates a bad attitude. That's just an excuse.

Back to your original statement: "An employee who is overqualified is almost as bad as one who is underqualified." An under-qualified employee is unable to do the job, while an over-qualified employee is able to do the job. The over-qualified employee can't be almost as bad since he or she can do the job.

We have different management views. Not worth debating. You would hire a doctor as an engineer. I wouldn't.

We have a very overqualified employee. He headed buying for a company with 900+ stores. He came to work for us at the time we had 2 in his area of expertise. Even now we only have 36. He even worked the counter some and trained store employees and has been incredible. He left his previous employer because of things going on that he felt were improper. He was going to work a couple or three years, then retire again, but 7 years later he says he has no plans to ever retire. Our Chief Stew and our Chief Engineer previously worked in those positions on boats from 200' to 400' in length with large groups reporting to them. They didn't want that life anymore.

The fact is though that the OP isn't overqualified. He's misqualified. When it comes to the job as a marine engineer, he's underqualified.

Well, I wouldn't hire a doctor as an engineer unless he had marine engineering training, which he doesn't. Doctor neither counts for him or against him.

If this is something you're passionate about, go for it. I would happily hire crew member who is medically trained. That being said, you'll need to be realistic about a few things...
It's a long road to become an engineer. But a deck/engineer, which is basically a deckhand who assists the engineer when needed, is achievable.
The thing that raises the red flag for me, though, is the 3-months in an ER idea. That is not likely possible. A standard leave package is 2.5 days/month worked. This equates to 30 days/year. As a junior crew member, you're not likely to find anything more than this available. Over the past few years there have been boats which are offering full-rotation positions to junior crew, but I have feeling that those jobs will be eliminated in the wake of the Pandemic.

Here is my take- In today's world of health uncertainties and the mega rich quarantining on their mega yachts I think you would be very attractive as a crewmember.
I would focus on the 100 meter + to start with, I am sure you could make a deal to be the onboard doc and have some time working in the engine room or helping out in the dive department. I don't think the doc role would be take much time during your day. Many of these boats have or are having full medical suites onboard.
They most likely will have rotation, so if you need to work in an ER to keep your licenses current it could work out.
I think Ken is also right, a deck engineer aboard a smaller yacht, would also be a possibility, just need to find the right Captain that can see that the positives outweigh the negatives. There are a lot of upsides to having a full fledge Doc abaord.

Also a good suggestion. And for the OP- these last couple of posts are from large yacht captains.