Water Conduit

When a spell that deals lightning damage hits a surface of water, it can propagate in various ways:

• If the spell is a projectile, that targets only one creature, such as Zap Bullet, it explodes when it reaches the surface of the water, creating a 10-foot radius sphere on the body of water.
• If the spell is on a line area, such as Lightning Bolt, the spell's original trajectory ends, creating new branches of lightning inside the water, on a 15-foot cone in the spell's direction.
• If the spell is a cone, creatures inside the body of water have disadvantage against the saving throw.
• If the spell has affects an area on a radius, such as Ball of Lightning, the spell stops at the surface of the water, and the radius inside the water increases by half of it's original distance.

Creatures standing inside the new area must make a Constitution saving throw. On a failed save, they recieve the effects of the original spell. On a success, they only take half the damage done.

You may also rule that any creature covered in water has disadvantage on saving throws against spells or events that would make them staggered.