During a fight in a semi-flooded room, one of my players wanted to use Shocking Grasp on the water:
Lightning springs from your hand to deliver a shock to a creature you try to touch. Make a melee spell attack…On a hit, the target takes 1d8 lightning damage, and it can’t take reactions until the start of its next turn.
Wanting to reward him for a creative use of the spell, I allowed it, using an attack roll of AC 10 and hitting all creatures within 10ft of the spot (including caster).
Is there any rule for these situations? I want to reward creativity without unbalancing spells that are meant for single-target use.
Best Answer
Specific
The first question you need to ask is if the creative use of the spell agrees with the spell requirements.
PHB, p. 275:
This spell requires you to deliver on a creature, and not an object.
Now, this doesn't really answer your general question about creative use of spells, but it does give guidance on the specific example you gave.
General
Generally, The Rule of Cool* should encourage creative solutions, as long as they fall within agreement of the rules. Ultimately, there is no real guidance as to how to handle this as DM other than 5e giving the DM ability to adjudicate decisions. You've got the freedom, you've got the power, just come up with something reasonable that is fun for everyone.
Also consider other effects someone's creative idea may have. They may want it to work one way, but if you see additional effects that could happen, then feel free to add them.
In the case of something like Shocking Grasp, instead of targeting the water (which is not a valid target), the player could target a creature who is in water and have the electrocution spread out out from your target to effect anything in the water (foes, friends, and possibly/probably self.)