Create water in their lungs or destroy the water in their veins. It says create water in containers within range. Lungs could count right?

I once used this to drown a dragon to death in my first move by creating water in its lungs p.s the mouth was open so my DM counted it as a open container 

A lot of things are technically containers, so to avoid debating on technicalities I usually just say no to that. Other than anything players can use against monsters monsters can also use against players, and they wouldn't really like that.

If you need a guard to leave the room, just fill his bladder! You could also instantly cure constipation. 

Just so everyone knows and I hope my maths is correct here.

If you can cast this spell 4 times per long rest

It would take you 7,442,922,374 years of constantly using destroy water and long resting in order to destroy all water on earth.

Given that no one else is on the other side of the beach using create water.

EDIT:

The calculation for a short rest was requested due to Warlocks only needing a short rest to reset spell slots

This would take 930,365,297 years to complete the task of destroying all water on earth.

I hope this helps you in any way that this kind of information could help someone.

I just love this whole idea.

But hey, did you take into consideration that a level 20 warlock can upcast it to level 5 all 4 castings?

Just checking.

If this was cast completely underwater in a large enough container, would it create a vacuum or would it fill with air? And if there was a sudden 10gal vacuum in water, what would be the amount of energy released?

Quote from Nostromo1975 >>

Nada. Elementals are creatures. This spell only affects water not creatures. 

 However, you could use the Create Water option on top of a fire elemental.

Maxie from Team Magma

Don't know if this helps, but I calculated out the average water elemental (being a large creature, we'd say roughly less than a 15ft cubed entity of water.  Doing math, we find 15ft^3 water is roughly 112 gallons of water.  A water elemental has roughly 114hp, so I figure for every gallon destroyed it would take away 1 of its hp.  So at 1st level, this spell can deal 10 necrotic (because of the destroy part) damage to the elemental.  This is for Water Elemental, not for Water Weirds.

If it was me and a Player wanted to Use this against a creature, I would do a series of things, 
1)intelligence Check to know where to target the spell inside the lungs DC being scaling to size of creature.

2)Creature make a Constitution save to be unable to act for one round expanding the water in the creature on a fail or similar

3) Prevent creating water, or destroying water in a creature that is Medium, or Smaller from Suffering full damage.(size restriction lungs couldn't hold that much water. 

4)if the attack is successful, 1d4/foot high of the creature. destroyed water replaces it with air, not a vacuum, your destroying the water not matter, air replaces.

Just my Quick Thoughts on it. 

if your splitting the water into o2 and h then you can make a bunch of flammable gas 

Is there a reason this couldn't be cast as a ritual? 

so I was looking and it says that a drop of water when destroyed creates a few grains of sand. So, there are ruffle 75,708.24 drops of water in a gallon times that by 10 then by 5 because it says few and few is 3 to 7 and in the middle it is 5. Then every grain of sand is ruffly 0.0625 mm so put that in your equation and you get 236588.25 then change it into meters from millimeters you divide by 1000 then divide by 3 to get it cubed and you get 78.86275. 1 cubic meter of sand weighs about 1600 kg. Then if you turn it into lb you get 3527.396 then times that by 78.86275 you get 278180.148899 lbs of sand to drop on your enemy.

How would y’all rule the temperature of the water? Same as the ambient air temperature? (What if they’re somewhere hotter than 100C or colder than 0C?) Any temperature at which it would be unable to cause damage? Being able to run a hot bath (if a very shallow one) would be a pretty sweet trick.

If a player ever suggested using this to fill a creature's lungs with water and kill it...  I would put it to a vote of the players.  Because if it is legal for a PC to do, then it is legal for a monster to do to a PC.  First one to vote yes is the first one to have it done to them.  That's the thing about "creative" (i.e., stupidly overpowered) uses of low-level spells...  if one person can do it, everyone can do it.

The spell never says how fast the rain falls, so you could technically convince your DM to let it fall as quickly or slowly as you want. You could probably get your DM to let it inflict massive amounts of damage if the rain all fell in less than a second (would be more effective the higher the level of the spell slot used).

In extreme cold. Water freezes really fast. I had the idea of making the ice/hail do falling damage from the alternate form of create water. (30 ft cube). Here was his (edited) answer:

So for create water dealing damage in extreme cold. You are gonna have to cast it a minimum of 30ft above the targets head to freeze. More distance will be needed for greater amounts of water.
The effect will be 1d8 bludgeoning damage. Each level above 1st gets an additional d8 of damage provided you cast the spell high enough above the target to freeze all the water.

Also I thought about dropping water in a silver/white dragon's mouth when they go to breath attack. The water should freeze and then create an barrier that the breath cannot pass through.

This spell was amazing in 3.5 and now its 2 spells in 1.

I think anyone who thinks that this spell applies to the insides of people's lungs (regardless of the pedantics about whether it's a container) are forgetting the basic PHB spellcasting rules:

1. The spellcasting rules state that, when you cast a spell that has a target, you require a clear path to the target, so it can't be behind total cover. I think it's clear that, in create or destroy water, that target is the open container in which the water is created or destroyed.

2. Combat rules state that a target has total cover if it is completely concealed by an obstacle. Assuming a creature's lungs do constitute an open container, they are still completely concealed by an obstacle (i.e. the creature) unless the creature's lungs are exposed. This means the creature's lungs (i.e. the open container) are behind total cover.

Since you cannot choose a target that is behind total cover, you can't choose a creature's unexposed lungs as the open container in which to create or destroy water. This obviously then applies to any internal organ that is unexposed, like the bladder or stomach. You might be able to fill their mouth with water though, causing them to be temporarily silenced and possibly choke on the water - that depends on whether an open mouth counts as an open container.