I'm doing that now with a 150 Mercury 4 stroke and an enertia 15p. I just took "The Hermit" and his friend out on a 120 mile trip. Full live well 35gallons, 90 gallons of fuel, and 3 people. I would pick the Merc 150 over the yamaha 200, b/c the Mercury 150 torque curve peaks at 3000rpm vs. yamaha 200 which peaks at 5400rpm. In head to head comparisons, I've heard the 150 Mercury will out perform both the 150 and 200 yamaha. I'm not going to post the fuel burn numbers, b/c no one will believe them. I will say it's better than 3mpg and we were plugging through slop most of the day cruising 27/28mph.
