Can a Water Elemental freeze, and what happens if it does?
Since it consists of water, it seems plausible that it can freeze. However, when this came up in our game, the DM ruled that freezing the Water Elemental would restrain it, and since it is immune to the Restrained condition, it cannot freeze. It's not impossible to imagine a frozen Water Elemental that is still not restrained in the sense of the condition in the game, but I actually liked this ruling and it was fine for our game. Still, I'm also interested in finding a RAW answer to the question.