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.
Best Answer
I would estimate the skill of the creators spellcraft and make a roll each time the item is used underwater. This would make it unreliable, sometime it would work and sometime it would not. This would be more fun and in the spirit of the rule.