Being a mechanic is Zero percent of being a captain.
That said...I am the first to say boating isn't all that hard and is only as dangerous as you make it.
Seeing impending danger is a product of experience.
Heading for the Bahamas is a moderate responsibility...but even beginners with enough coaching can do it. Blue water excursions require tens of thousands of miles of blue water experience...aircraft carriers don't count.
A big roll of the dice?
Your example is similar to the best jet engine mechanic becoming a pilot in 6 years of really part time work.
Would you take your family on a flight with someone with those qualifications?
I might...but I would have to know the guy became rabid on preparing himself for the responsibility.