The measured distance, say 500 meters certainly does make sense. you could then just state your time like in swimming or running.
How accurate is a gps? Measuring distance over water and getting your gates set on the water can't be a very exact science. If you wanted to go all out you could do gps speed, timed speed, and average the results. not sure if it is possible to train a radar gun on a craft for a full 500 meters and save all that data.

That video of the tornado sure shows alot of wasted energy. if only it was all going towards forward motion.