I have weird ideas today. What happens when you turn on a HID bulb under water? I would try but for that I like my bulbs too much

KA-BOOOOOM!!! 

I mean to stick it under water and then turn on, not plunge a fully warmed bulb into the tub  Of course that would explode.


I think it won't run up fully, I was curious if it would run up at all... Maybe, a HPS bulb would stay blue-white and not turn orange, but I don't know.

You mean the whole intact bulb, or only the naked burner?
In both cases you would need to use deionized water, otherwise it would cause strong electrolysis and short circuity arcing.
If whole bulb, HPS and CMH would most likely run normally, as in the outer is a vacuum, so the arctube is cooled down only by radiation, so no influence of the outer's temperature. Lamps with gas filled outer would run too cool, so the pressure would be lower then normal.
If naked tube, i thing glass arctubes would run as after startup, when the water cover whole surface. But when vapor bubbles form, the arctube would likely crack doe to large thermal gradient.
And on ceramic tube would most likely fail seals - as they are chemically very sensitive...

I meant to stick the bulb into the water but leave the base above water surface... But if nothing interesting happens, there's no point in doing this

I once filled a bulb with water and fired it up. It never came to full brightness. It also took a very long time to reach any level of brightness. As for sticking a whole lamp under and turning it on I would think it would come to full brightness. Though I cant be sure as I have not tried that.

There were some here that got flooded in a severe storm, and were working happily underwater, at least until the supply fuses blew as the ballasts shorted out.

i Filled a GE Mercury Vapor Lamp with Water and it didnt turn on at all!

i Filled a GE Mercury Vapor Lamp with Water and it didnt turn on at all!

As the water was probably not deionized, so electrically conductive, it shorted the auxiliary probe with the adjacent main electrode, so it didn't have it's starting discharge...