Storm King's Thunder
Boiling, Freezing, and Shocking in Water
A practical guide to what magic does to liquid
AS far as I'm aware, 5th edition does not have any rule sets when dealing with the effects that are caused by fire, lightning, or freezing. So, here is what I will say on the matter.
Whenever an attack deals damage to a square that is 'water', a soft steam begins to form out of the location for a short amount of time. The area affected by the boiling is considered lightly obscured for half damage dealt rounds.
When attacking a creature that is within a water square with an ability that deals 'lightning' damage and requires an attack roll, the attack is considered to be with advantage. If the spell is not an attack roll, but rather a spell saving throw, the targeted creature(s) have disadvantage on the saving throw.
When you attack a water square with an attack that deals cold damage, the area becomes difficult terrain. You can choose to ignore this effect (Similar to ball bearings); however doing so, you must make a DC 10 acrobatics check to maintain your balance.
If you are attacking an area with a creature in it, the creature must make a DC equal to the attack roll halved or become grappled.
The ice has an AC equal to the damage roll, with a immunity to psychic damage, and HP of one. The creature effected by the ice can also attempt to escape by making an athletics check equal to the ices AC.
The Ice naturally dissipates after damage dealt halved rounds.
Attacks with fire-based abilities have advantage against the wall of ice.