To tell you the truth, you don't really need a submersable water heater. In fact, I don't really think it will work anyways.
Try to follow my reasoning...
If the water in the tank gets hot, by the time it reaches the nozzle itself, it will have cooled off. The water does not travel at that great of a rate to get from the bucket to the nozzle while still warm. (Remember, the nozzle rate of water use is only enough to lay out a "super-fine mist" for the enclosure). My line is about 6' feet long, and I know that in the one or two minutes of misting that I have it set for, the water from the line is the only thing that will get used. The water from the bucket will NOT travel to the nozzle tip in this two minutes of misting.
Your best bet would be to warm the water WITHIN the hose itself. Preferrably where it's closest to the nozzle.
What I did was lay the hose over the side of the basking bulbs dome. Not the ceramic part, but the black metal dome itself. The plastic will ot melt as the water will absorb most of the heat from the dome - hence keeping it nice and toasty before getting misted.
Use common sense. Don't lay the water hose over exposed wires, or keep the nozzle so close to the dome that if the connection from the hose to the nozzle leaks, it will spray onto the ceramic part of the dome. (Duh)!!
Anyways, that's my opinion. Try it out FIRST...then see if you really need to invest your hard-earned shekels on a "submersable water heater". (Is that what they're called)?
Like I said...I don't think it'll work anyways. But then again, I may be wrong as well. (Naaaaww...I don't think so)!! Ha ha.
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In Christ: Raymond