I have been looking to do this as well, as a sculpture for next year's
Burning Man festival, actually.

The challenge is keeping it lit. What I have found is spa heater igniters.
They use a thermocouple to decide if the flame is out, and then
automatically spark to relight it. So my plan, (not yet tried), is to take
one small pipe off of the underwater feed and direct it to the thermocouple.
If it stays lit, it lights the rest of the gas, if it goes out, the igniter
lights it and the pool of gas on the water's surface as well.

Any thoughts? I am pretty committed to propane, I think, as it is easy to
get, transport, etc.