Interesting premise there Baker; the way you stated it anyway. "A truck gets up to speed and then the load decreases significantly." By "load"... You do mean the load of hp. required for accelerating the inertia of a truck's dead weight... correct? Once up to speed the truck engine has to overcome other uniquely variable loads [such as tread designs, tire pressures, winds, bearing designs] than before due to natural airflow blockage or assistance and faster tire rotations and more bearing/grease rotations/frictions.
Therefore, although considerably different in many respects/aspects of friction and natural property blockage/assistance during acceleration and after attaining cruise speed, the overcoming inertia "load" during acceleration for a boat engine also decreases significantly once hitting cruising speed... correct? That is of course depending on speed reached, how quickly speed is reached and what design hull is riding in what manner either in or over the water.
Of course each "load" factor for land or water craft, of accelerating weight and then maintaining speed, has included in its formula just how fast to accelerate to reach and maintain a desired speed.
https://www.google.com/imgres?imgur...ved=0ahUKEwiC0KWwpL7XAhUr6YMKHeNWBn0Q9QEIKjAA