The motors will get to 90 to 103 degrees on a very very hot day, but no worries they motors are rated for 400 degrees, overheating is not an issue for those motors . We fly over the water almost everyday and we have never had any issues , and we try to fly as close to the water as possible to gain some momentum shots of a nice sweep up.
You can see are videos at phantomrain.org This is - 6 degrees over the lake skimming and sweeping, GPS remains sold.
Where did you come up with 400 degrees? This is insane.
According to my 30+ years in RC's, 160-180 degrees is about all you really want. If higher something is off and magnet damage might occur.