The y-intercept of a graph that plots force (y-axis) versus acceleration (x-axis) describes the relationship given by Newton's second law of motion, \( F = ma \), where \( F \) is force, \( m \) is mass, and \( a \) is acceleration.
In this scenario, if we rearrange the equation, we can express the force as a function of acceleration:
\[ F = m \cdot a + 0 \]
In this case, if we consider acceleration to be zero, the force exerted by the motorboat would also be zero. Therefore, the y-intercept, which represents the value of force when acceleration is zero, would be:
zero
So, the correct response is "zero."