"It's difficult to try and
"It's difficult to try and give you a simple answer.
There are two distinctly different versions of the Mariner line.
One, as you note, was completely manufactured by Yamaha - the last few models were sold as 1994 model year motors.
These were not a unique design nor were they made to Merc specs, they were simply models that Yamaha had designed and marketed themself which were simply "branded" as Mariners.
In most instances, these were discontinued by Yami before they ever hit the dealers as "new" Mariners. So that 1990-whatever 30 horse was really a 1978 "A" model Yami.
In the Yami/Mariner line the largest horsepower produced was the 60 horse (last model year was 1983), with the 40 "C" model being the most prolific.
Although well built, they were using "non-current" technology which allowed them to be sold at a lower (attractive) price point.
The initial plan was to sell these in the Asian market with "real" Mercs being sold in Europe/N.A. and Australia through their mfgs located in those areas.
However, the Yami Mariners popularity quickly spread (most likely due to the lower price and decent quality) and many made their way to North America.
The "other" Mariners were simply a Merc with the Mariner decals and paint (probably trying to capitlize on the success of the Mariner line, particularly in Europe).
Yes, through a joint venture Yamaha provided a considerable chunk of the parts, but it was a very much a Merc.
All Merc powerheads that are of the "headless, split block" design (and some of the ones "with heads") were manufactured by Yamaha. As far as I know, Yamaha built these exclusively for Merc and I am not aware of any Yamaha outboard that ever used this headless block.
Yamaha also made most of the gears for Merc. Again though, they are not interchangable with any Yami product out there.
Merc simply had Yami manufacture this "stuff" for them. (eventually this partnership fell apart when Merc accused Yami of "dumping" into the U.S. Market, and although they continued to source some parts from Yami, eventually they crawled into bed with Tohatsu, who has now effectively taken the place that Yamaha once had).
On the Merc/Mariners, the ignitions were Merc, the carbs were Walbro's and the gearcase itself (and other machined parts such as the exhaust plates, intakes etc) was totally Merc design.
So, there is no particular "year" cut off when before was Yami and after was Merc - it is on a model by model basis and in some cases, the 40 for example, between 1989 and 1994, you could get them in "both flavours"...."
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)