one of your points is certainly valid the others I would suggest are questionable, because: A) Glow plugs are designed for immersion in a very much more hostile environment than water! B) cavitation and oxidization occurs at vastly higher temperatures that the water would allow,which in turn would extent the life of the glow plugs, they are sealed internaly to a greater design integrity than any water heating element, and made from extremly high grade nickel alloy, however you are correct that they use a lot of current (even more if immersed in water due to the lower temperature opperation) and would best suit being used durring engine running to minimise charge loss, but never the less they work very well indeed if their voltages are kept correct.