Answer:
Initially it was a joint venture/ import of Yamaha engines rebadged as Mariner by Mercury. Later Mercury built all of them. This was certainly true of the smaller engines as I remember my dad not buying one as he wanted a genuine Mercury to replace our red band in the mid 70's. Rumors that the lower casings of earlier mercs where susceptible to corrosion in salt water could have prompted the move. Also possible is that in the 70's Yamaha was looking to expand and it was a tax/exchange rate thing as they where introduced in Australia and Europe before US.
In the end they are now one and the same thing.