Post some info about the boat itself and the steering system.
It sounds like you bought a new motor for an existing boat. The problem may not be the motor. Its Most likely something in the steering.
When my steering got so stiff that I needed two hands to turn it, I found it was the cable from the helm to the power steering ram at the engine. I changed the cable, now I can turn it with one finger.
I tried everything, grease, etc. Then I disconnected the cable from the power steering ram and the steering was still super stiff. New cable and what a difference!