A fire elemental cannot enter water or any other nonflammable liquid. A body of water is an impassible barrier unless the fire elemental can step or jump over it.
Fire elementals speak Ignan, though they rarely choose to do so.

A fire elemental is a fierce opponent that attacks its enemies directly and savagely. It takes joy in burning the creatures and objects of the Material Plane to ashes.
Burn (Ex): A fire elemental?s slam attack deals bludgeoning damage plus fire damage from the elemental?s flaming body. Those hit by a fire elemental?s slam attack also must succeed on a Reflex save or catch on fire. The flame burns for 1d4 rounds. The save DC varies with the elemental?s size (see the table below). A burning creature can take a move action to put out the flame. The save DC is Consitution- based.
Creatures hitting a fire elemental with natural weapons or unarmed attacks take fire damage as though hit by the elemental?s attack, and also catch on fire unless they succeed on a Reflex save.

Fire Elemental Sizes
Elemental	Height	Weight	Burn Save DC
Small	4 ft.	1 lb.	11

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System Reference Document -> List of Creatures (SRD)