Differences in water chemistry, density etc can indeed affect speed, but I have never seen more than a few mph - certainly not 15 mph. Full-sized boats see the same, never a 30% difference. I have run on water in five different states and a dozen ponds too.
I set many SAW records with 05 motors, usually pulling between 400 and 1200 amps depending on the cell count. Never melted an endbell or burned up a motor, but then I seldom ran more than a minute or so. I won the last-ever APBA P Sport Hydro National Championship pulling an average of 750 amps for over two minutes with a single 05 motor on a gearbox. These were all with NiHM cells, the setups ran for years.
I raced Ed Hughey at a SAW in Huntsville some years ago, he ran geared brushed motors and I ran direct drive brushless. No contest, but Ed was very gratuitous, a real gentleman.
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