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Asking on behalf of the group's Wizard. He wants to electrocute the BBEG and have the spell deal extra damage through clever use of game mechanics. As a DM, I am all for it, as long as the rules somehow support it.

Create Water. You create up to 10 gallons of clean water within range in an open container. Alternatively, the water falls as rain in a 30-foot cube within range,* (PHB pg. 229)

Is this something you could cast, and either cast Lightning Bolt (8d6dmg) on the target being rained on(on your next turn / action), or use Shape Water to form a cube around the BBEG, and then Lightning Bolt to the face?

I personally think something being wet is conductive, which I might say means that the target being rained on would have disadvantage on the saving throw. My player really wants to try and argue that being wet yields a Damage Vulnerability (2x dmg), therefore causing lightning bolt to deal (8d6)*2 lightning damage.

Is there a rule I haven't found in the PHB about this already? I do really want the player to feel cool, especially since this upcoming week there is a BBEG confrontation and this might turn the tide for my group.

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Imposing Vulnerabilities is not (strictly) in the DM's discretion...

Vulnerability is a very strong mechanic, and I would be hesitant to impose vulnerability unless it's written somewhere in the creature's stat block. It doubles damage received, and from the scenario you describe, Lightning Bolt with Vulnerability looks to deal about 56 damage on average for a use of one 1st-level spell (Create water) and one 3rd-level spell, all in two Actions. That is just too strong, taking out a third of an average CR5 creature's HP in one turn (not to mention you can do it again on the next turn, because the creature is still wet).

That mechanic is ripe for abuse (especially when you turn it around, and use it against the players) and I wouldn't recommend it.

... but Dis/Advantage is!

Instead of Vulnerability, give the creature Disadvantage on the Dexterity Saving Throw against Lightning Bolt. It is totally in the rules, and is balanced:

(DMG 239)

Advantage and Disadvantage

... Characters often gain advantage or disadvantage through the use of special abilities, actions, spells, or other features of their classes or background. In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result.

This way, the damage is still within sane levels, but the likelihood of the creature saving for half damage is reduced.

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    \$\begingroup\$ This is precisely how I'd handle it. Advantage is most commonly used for skill checks, and frequently for attacks, but it's rarer to see it applied to saving throws, and I think people forget that that can happen as well. The player needs to remember that they describe their actions, while it's up to the DM to decide the consequences, and the advantage/disadvantage mechanic is one of the DM's primary tools for representing things that tip the scales like this. \$\endgroup\$ – anaximander Jan 25 '17 at 9:53
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    \$\begingroup\$ It also rewards the player for creativity which is almost always what you want to do as a GM. \$\endgroup\$ – Paul Jan 25 '17 at 11:30
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    \$\begingroup\$ @Paul Applying a damage vulnerability like the player suggested would also reward them. The difference is that the game is designed for advantage/disadvantage to be applied to pretty much any d20 roll and not break the balance. \$\endgroup\$ – anaximander Jan 25 '17 at 16:22
  • \$\begingroup\$ @anaximander I feel like that aspect was covered in the answer already. My comment was intended to be additive to what he already said, not comprehensive. \$\endgroup\$ – Paul Jan 25 '17 at 19:54
  • \$\begingroup\$ +1 for mentioning granting disadvantage on a save! That's how I would rule it. \$\endgroup\$ – Christopher Jan 25 '17 at 22:35
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I don't know if there's a rule for this, but looking at this realistically...

Lightning (as a damage type) isn't normal electricity

We can infer this from it working equally well against ungrounded (eg. flying) opponents, foes wearing insulative material and so. And, of course, the lightning not taking the path of least resistance. If you choose to allow your players to apply normal laws of electric conductivity, that's your call, and if the players think it's cool you might just go along with it, but beware: you might be opening a can of very weird electric worms for them to explore. If you want to impose a penalty, I recommend Daze413's suggestion to use a disadvantage/advantage instead of a vulnerability.

Also note that being submerged doesn't magically make electricity sink in better. In fact, with conductive water surrounding your presumably less-conductive BBEG, the electric flow should take the path of least resistance around the BBEG instead of through them, possibly even making the lightning burns less severe.

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    \$\begingroup\$ I was considering your last sentence as a comment, except for, welll magic. Which might not make much (or rather, too much) sense in the game, but it does make for a great DM rebuff. \$\endgroup\$ – Hennes Jan 25 '17 at 8:52
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    \$\begingroup\$ @Hennes, I see what you mean. Either lightning is magick and ignores non-magical resistance, or it is realistic and well, behaves realistically. Water doesn't help in either case. \$\endgroup\$ – kviiri Jan 25 '17 at 8:54
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    \$\begingroup\$ If the water created by the spell is pure enough, there is actually a distinct possibility that it conducts electricity worse than the BBEG. It is the impurities in "common" water which can make it a good conductor. en.wikipedia.org/wiki/Purified_water#Electrical_conductivity \$\endgroup\$ – eirikdaude Jan 25 '17 at 11:50
  • \$\begingroup\$ @eirikdaude the impurity being the BBEG himself. \$\endgroup\$ – daze413 Jan 25 '17 at 13:33
  • \$\begingroup\$ @eirikdaude You're not wrong, but since we're talking about a spell which somehow (magic!) forces arcing (and thus presumably ionization) along a specific path, it's very hard to say how that's applicable. If it works by e.g. ripping molecules into ions, then the conductivity of water is irrelevant and you're looking instead at the conductivity of some sort of shaped, low-temperature, ionic hydrogen–hydroxide pseudo-plasma. I am unaware of any good descriptions of that substance's conductivity. \$\endgroup\$ – fectin Jan 25 '17 at 20:29
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There is nothing in the PHB that you missed. The rules for damage types start on page 196 and say nothing about assigning resistance/vuilnerability. But if you look in the Monster Manual you will see that water elementals (p.125), not only doused, but made from water do not have vuilnerability to lightning. And another creature from the elemental plane of water, a marid (p.147) even has resistance against it. Based on this it would be strange to assign vuilnerability in this situation.

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As the reason for the assumption that lightning bolt + water is more effective is based on real world physics my answer will be, as well.

Water is only conductive if it has minerals or other "pollutants" in it. Pure H2O is not conductive. We do not know whether create water produces pure or mineral containing water (now called normal water).

Pure water

Being rained on by pure water does little but as clothes and the floor both always have stuff on it the water will soon become mineralized. But still not a lot of effect on a natural lightning bolt.
Being encased by a pillar of pure water will give you insulation leading to less damage taken.

Normal Water

Normal water is conductive. Being rained on might make it harder to escape the lightning bolt.
Being encased by a pillar of normal water would give the lightning bolt a path of least resistance that bypasses the body but would generate lots of steam which could cause fire damage.

Conclusion:

Combining create water with lightning bolt might be a reason to give disadvantage but not to increase the damage.
Combining shape water with lightning bolt should convert half the damage to fire but neither increase the damage nor give disadvantage.

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A creative use of water and lightning I've personally done on several occassions, much to my DMs approval, was to aim my Lightning Bolt at a pool of water that several enemies were standing in, asking my DM if we could somewhat reduce the damage of the spell, but affect all of the enemies in it. And he perked up and agreed, not even halving the spell's damage, but just removing a couple of dice from it, and all the dice affected each enemy.

Later I proposed a new homebrew rule, which we then polished together, and the final version was that for every 10x10ft of water (thin layer or a shallow puddle) or 5ft cube in deep water, the spell deals 1 fewer die of damage, but to all targets in said pool of water. We also agreed that the sphere created by Water Sphere spell subtracts 1 die from the bolt per target inside the sphere, for balance sake, but it's still a mighty combo.

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    \$\begingroup\$ Hello and welcome! Take the tour for a quick site introduction. Answering old questions is perfectly fine, there is even a badge for that. Thank you for participating! \$\endgroup\$ – Sdjz Aug 8 '18 at 12:00

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