Final answer:
The force at play when a water-skier is pulled by a speedboat at constant speed is primarily the tension force transmitted through the tow rope.
Step-by-step explanation:
When considering a water-skier being pulled by a speedboat at constant speed, the primary force at play is the tension force. This force is transmitted through the tow rope connecting the skier to the boat. While frictional forces and gravitational forces are present, they are not the forces responsible for the skier's constant motion. There could be centripetal force involved if the skier is moving in a circular path, but without that specific context given, we typically focus on the tension acting along the rope.