Sounds like a diesel classThe old discussion, is it torque or Hp? Regardless of how you look at it, it is torque that turns the pump. The ideal situation is having an engine that generates peak torque at the maximum Rpm you intend to run and match the pump impellor to absorb that level of torque at that Rpm. That results in the greatest volume of water moved or the greatest amount of work accomplished per unit of time or Hp. If you begin to add the variables of pump volumentric efficiency and flow capacities of the engine it gets a lot more involved. If you have two pumps moving the same amount of water the pump with the larger impellor running at a lower speed to accomplish the same amount of work will be more efficient. It is just the way it is. The challenge is you do not have many boats that can support an engine making 1,000 FtLBs of torque at 3,000 Rpm turning a 14 inch impellor and still go fast. So you build an engine making 600 FtLbs of torque at 5,500 Rpm turning a 9 1/2 inch impellor and have commonly available parts to do so.