In a graph of acceleration versus net force, the slope of the graph represents the inverse of the mass of the boat. According to Newton's second law of motion, \( F = ma \), where \( F \) is the net force, \( m \) is the mass, and \( a \) is the acceleration. This can be rearranged to \( a = \frac{F}{m} \), which implies that the slope (change in acceleration with respect to change in force) is equal to \( \frac{1}{m} \). Therefore, the correct answer is:
the inverse of the mass of the boat.