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.
If the character is casting the spell, the above rule is straightforward. But what about magic items that produce spell-like, fire-based effects? For example, a flaming sword, salamander's cloak or blast globes.
For spell trigger magic items, like staves and wands, it makes sense to use the Spellcraft skill of the item's wielder. But for command word and use-activated items, that doesn't seem right to me.