The plan was to allow the mapper to set the frequency of bubbles spawned, the extent of the area in which they are spawned, and the speed at which they are 'shot' upwards. I stumbled on a problem though, the values I would enter in Doombuilder were not having the same effect as if the same values were 'hardcoded' into the Decorate lump. I was tired so I gave up.
If you can get it working feel free, it can be a joint-entry
Oh, and I noticed strangely that with additive or translucent rendering, the bubbles always remain grey despite being in a coloured sector which looks odd. Dunno why it does this. Gez's HPack foggy projectile fix?