As the old adage goes "water and electricity don't mix" because in real life water has impurities in it that make it super conductible. So what would happen in D&D if, for instance, a sorceress cast shocking grasp on a creature underwater?
Would the spell not work? Would it affect the caster as well? Would it spread and hit everything under water?
Shocking grasp in particular specifies that "You have advantage if the target is wearing metal armor" which indicates that they are taking conductivity into account.
What about call lightning or chain lightning, would casting a higher level spell change the outcome?
Best Answer
Lightning damaging spells do not behave differently in water
The various lightning spells you mention, shocking grasp, call lightning and chain lightning, none of them mention interacting with water in any way, so they don't. Spells only do what they say they do, although a DM is free to rule otherwise (also see the end of the quote at end of my answer). The section in the PHB about underwater combat (pg. 198) also doesn't mention anything about lightning spells behaving differently in water.
Regarding upcasting spells, any spell that has addition or different effects when upcast has a section that starts with At Higher Levels, and no spell mentions how lightning damage is enhanced by water when upcast.
Although this quote from JC is about fire spells underwater, I think the same reasoning applies to lightning (from Does Fireball evaporate water?, borrowed from this answer):