Right now I have a 9.9 on my Tracker 1654. (460# bare boat, + quite a bit of added weight). Sure won't set any speed records, but it works. 12 mph alone, 9 mph W/a passenger. That's on a lake.
In a river with heavy current I would want to go bigger, maby a 15 hp or more. With only me in the boat I barely made it up a river we fish with the 9.9. Also have a 30 hp for the boat, and if someone were with me I would throw that on.
Years ago, 2 of us in an old barebones mirrorcraft deep v couldn't make it through a riffle (whitewater) with a 9.9.
A 9.9 will work OK in flat water and some current, but with 2 people and heavier current they may not have the power that's needed.
Need to add this:
Also have used a little 4.5 merc on the river, on a 17' starcraft. It got me to where I wanted to go but never would have pushed that boat through any type of riffle.
It all comes down to what type of water you are attempting to navigate, and how much weight is in the boat.