I found this. It's from 2006, but as far as I can tell is correct for today.
Quote:
Drive Manufacturer
Who Actually Builds the Drives
Bombardier/OMC
Johnson and Evinrude
Under both builders, from the powerhead down, the major differences in Johnson and Evinrude are the cowls and paint color.
Until 2006, Suzuki manufactured the Johnson four strokes up to 225 HP.
For 2006 Suzuki only makes the Johnson four strokes up through 30 HP, Evinrude produces direct injected two strokes (E-Tec) 40 HP and above.
Nissan
Nissan outboards are built by Tohatsu
Honda
Long ago, some Honda three cylinder powerheads were mounted on Mercury drive legs and props. For the last several years, Honda has built its full line of outboards.
Mercury Marine
2006 model Mercury four strokes from 2.5 through 30 HP are built by Tohatsu.
Before Yamaha entered the U.S. market, Mercury's Mariner line was built by Yamaha.
Last year (2005), Mercury four stroke powerheads from 25 through 225 HP (except the Verados) were built by Yamaha. Several of those models used Mercury drive legs, electronics and EFI systems (replacing carbs Yamaha used in similar sized engines).
In 2006 models, Mercury four stroke powerheads from 40 to 225 HP (except the Verados) are built by Yamaha.
Mercury's agreement with Yamaha ended in March 2006, at which time Mercury is expected to phase in more four cylinders (75 to 115 HP) based on the Verado engine block, but without supercharging or intercooling.
Ed