When a pump is submersed, every single BTU of heat produced by the pump ends up in the water. There is no way to avoid that.
What you want to look for is watts versus GPH and head height. There is no set way to look at these figures, so you have to lay out a few differnet pumps, and develop a ratio of GPH, Head Height, and watts.
The more GPH and head height you can get for a given number of watts (or conversely, the fewer watts you can pull for the same GPH and/or height height), the more efficient your pump, because more watts of power input are converted to waterflow and less watts of power input are converted to heat.
Hope that helps at least some..