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 meanmeant for single-target use.