This is why we test things, too much theory, not enough practice!
I did a performance chart when setting up my Precedent Perkasa, over on the Warships bit, (Perkasa to Brave Swordsman). I started at 4.8v both in and out of the water. The Leopard motor was listed as a 2 to 4S designed, while the Ripmax Quantum II 20 was listed as a 4S. So starting at just above 1S, both motors were happy to fire up and rotate both in and out of the water, with torque.
So, I don't know if my description was materially accurate, but I stand by the 'in the water' tests disproving this voltage thing. I still stand by my thoughts that the recommended voltages of most of these motors are aimed at getting an aircraft off the ground with the recommended prop. Even the Component Shop listing is recommending suitable aircraft prop sizes, even on the low Kv one listed a few posts back. A very large prop I might add, lots of useful boaty torque!