There are several reasons I can think of why you would find water/ice weapons in a fire-based dungeon: -Some other adventurer failed some/a long time ago, who was carrying the weapon you loot off of them. -There's a lake within the dungeon, which belongs to a water spirit whose home was invaded by the fire monsters. They'll give you that weapon so you can help fighting them off. -The dungeon is a mix of fire and ice (like Hailfire Peaks) and you use weapons from one segment to advance in the other. -The boss of the dungeon is of the fire element and put up a guardian to prevent the ice/water weapon from being used against them.