Any temperature over 155F can potentially damage the Nemesis magnets. Remember that is the case temperature, the interior magent temperature would be higher. You can limit motor case temperature by using a smaller prop, fewer cells, watercooling, a more smoothly-running boat, or by shortening your run times.
How hot are you getting? None of my sport hydros running Nemesis motors are watercooled, and even after a 5-lap heat on the big 1/6th mile course the can temperatures are typically under 145F.