Question....

[Dross]

Would a decanter of endless water on the gyser setting do damage to a fire elemental?

Yes,
"The force of the geyser deals 1d4 points of damage but can only affect one target per round."

Given that the SRD does not mention that the Fire Elemenatal takes extra damage from water, it's only 1d4 damage (as Green Slime alluded to).

Which creates the Q: what type of damage does the geyser do? Water and/or Force?

 

[atom crash]

Which creates the Q: what type of damage does the geyser do? Water and/or Force?

Bludgeoning? Untyped damage?

 

[RigaMortus2]

Bludgeoning? Untyped damage?

Untyped...

It would have to specify "The force of the geyser deals 1d4 points of [X] damage but can only affect one target per round" where [X] is replaced by a type of energy. Since it is not, it is untyped.