Here is my experiment:
YouTube - Water spark

I used a water heater to get hot steam. You can hear how the bangs become louder when the steam is released through the spark gap. This was filmed with a photo camera, that is why it is hard to see the flashes, but they were all there in all the impulses. If we used larger gap, the spark gets very weak after a while in hot steam and the bangs are not so loud anymore. I suppose that this is because the steam heats the electrodes and this makes some resistance in them. Anyway, I can confirm that when water vapor is ignited like this, there is a much louder bang than from the spark without steam, the flash becomes red/orange in color and you can see an explosion about 1.5cm in diameter. Also the electrodes indeed doesn't get hot