Related Question: Can magic items that deal fire damage function underwater?
As mentioned in the above question:
From the DMG p93:
Spells or spell-like effects with the fire descriptor are ineffective underwater unless the caster makes a Spellcraft check (DC 20 + spell level). If the check succeeds, the spell creates a bubble of steam instead of its usual fiery effect, but otherwise the spell works as described. A supernatural fire effect is ineffective underwater unless its description states otherwise.
For creatures that have damage reduction x/fire or regenerate damage except that done by fire, do they treat the bubble of steam the same as fire or not? eg Would a Water Troll regenerate damage from a fireball cast underwater?