You're right about the "trick": you have to keep in mind that "upstream" and "downstream" (just like "with the wind" and "against the wind") indicate competing or cooperating powers.
Suppose you turn the engine off in the motor boat. The boat would still move, right? It would go downstream, because the current is pushing it. Even though the speedometer reads "zero", the boat would still be in motion. If you turn the engine back on and head downstream, that pushing would still be happening; you would add
the speed of the current to the speedometer reading. If you headed upstream, you would subtract
the speed of the current from the speedometer, because the current would be pushing against the boat's progress.