I had a situation come up in my game tonight where a gunslinger opponent used powerful shot to push the druid, who was wild shaped into a fire elemental, off a ship into the ocean. The fire elemental specifically has an "ability" where if it has water splashed on it it takes 1 point of cold damage per gallon of water. The amount of water in a 10ftX10ft cube is thousands of gallons. Naturally this means it takes thousands of cold damage and kills the fire elemental form. My question is would this cold damage, which is only a result of a specific fire elemental vulnerability to water, carry over and also kill the druid?
In the moment I ruled it didn't, mostly out of fairness because that situation already really sucked for him, and I didn't think it was fair to kill him outright, even though turning into a fire elemental on a ship is a bad idea for a number of reasons, but I'd like to know for the future.
Best Answer
You could rule that the damage is applied 1 gallon at a time
The Water Susceptibility trait says:
It wouldn't be unreasonable to read that as saying that each gallon of splashed water deals its damage separately, in sequence, 1 damage at a time. This results in exactly the ruling you described: the druid in fire elemental form splashed with thousands of gallons of water will only take exactly enough damage to bring their fire elemental form to zero hit points, after which they will no longer have Water Susceptibility and will just be a soggy but alive druid.
Does a fire elemental actually die when it falls in water?
There is unfortunately some ambiguity in the Water Susceptibility trait. The two ways for a fire elemental to take "water damage" are having water splashed on it and moving in water. So, which of these two conditions applies when the fire elemental falls into a body of water? Is falling into water the same as having water splashed on you? Or is it moving through the water? Or is it both? I'm honestly not sure what the designers intended here.
Additionally, a fire elemental falling in water is not necessarily exposed to an entire 10 foot cube of water. The 10 foot cube is the space it controls in combat, not the volume of its body. On the other hand, if you assume that any water splashed on the elemental evaporates instantly, then the surrounding liquid water would quickly rush in until the elemental is exposed to enough gallons of water to kill it.
I could take this further, but we're already well into hypothetical "What If?" physics speculation territory, and I'm making a lot of unfounded assumptions. Ultimately, there's no real-life answer to how many gallons of water would be "splashed on" a fire elemental that falls in water. So the DM has to make a ruling, and that ruling should be based on fairness and game balance, not speculative fantasy physics. I've given one possible ruling above.