Good thing you didn't use the waterbath as a secondary QT either!
The Visitherm Stealth is supposed to turn itself off when it detects that the water level is low. I have done 50 % water changes while the heater is still plugged in. A few times the exposed part still felt warm to my touch. But nothing happened to it or the tank. It still keeps the temperature I set it at. If you used this heater in your waterbath, I don't think you would have to worry so much about evaporation. You would still check your water level, but if it happened to get a little low overnight, you wouldn't wake up to a burning smell!! 8O
P.S. I smelled a really strong burning smell once, but didn't see anything. It was so strong I was almost ready to call the fire department. Then I figured out that a moth torched itself in the halogen lamp. My goodness, did that smell! But now I know what it is if I smell it again, and I try to catch moths before they get themselves stuck in there!