There's nothing in the rule book that says you can't create water in a person's open mouth. Thus having 10 gallons instantly created in your mouth. If it doesn't blow your head off it will certainly send water hammering down into your lungs. Not the full 10 gallons but even a quart will be incapacitating.
So what would the saving throw be to avoid inhaling 10 gallons of water when you take a breath or open your snarky mouth in front of an enemy cleric or druid?
Per the rules creatures, nor parts of them that are still attached, are not objects (Object and Creature being mutually exclusive rules categories).
Because a creature nor any contiguous part of it is only an object, it cannot be a container, and therefore water in a creatures mouth (or any other part of its body) is not a valid target fort the spell.
(Note, this also applies for Destroy Water, which again specifies an open container)
So what would the saving throw be to avoid inhaling 10 gallons of water when you take a breath or open your snarky mouth in front of an enemy cleric or druid?
The fact the spell lacks information for such an action would be very good supporting evidence for not being able to use the spell in such a fashion
Even if you could do that, and there's strong evidence you can't, the creature wouldn't be incapacitated. It would just stop breathing. What happens next depends on your interpretation of whether having your lungs filled with water is "holding your breath" or "choking".
If it's "holding your breath", a creature can do that for a number of minutes equal to 1 + its CON mod, minimum 30 seconds. That's at least 5 rounds (longer than the average combat) to make sure the Druid who just filled its lungs with water has a bad day.
Here's the thing though: even if it's "choking", the creature still has a number of rounds equal to its CON mod (minimum 1 round) to get air before it falls unconscious. How long does it take to expel enough water from your lungs that you can breathe enough to stay conscious? I don't think there's specific rules for that, but "six seconds" isn't out of the question, particularly if someone helps you. So the creature loses an action coughing up water, and then is fully ready to go because there's no defined penalty for having a little bit of water in your lungs. Except now it's probably holding its breath (which it can do for at least 30 seconds), so that clever little trick won't work on it again.
In summary: this meme is pretty clearly forbidden by the rules of the spell, but even if it wasn't, the most you're likely to accomplish is burning one or two enemy actions to first aid. Not a particularly efficient use of a 1st level spell slot in my opinion.
There's nothing in the rule book that says you can't create water in a person's open mouth. Thus having 10 gallons instantly created in your mouth. If it doesn't blow your head off it will certainly send water hammering down into your lungs. Not the full 10 gallons but even a quart will be incapacitating.
So what would the saving throw be to avoid inhaling 10 gallons of water when you take a breath or open your snarky mouth in front of an enemy cleric or druid?
Create or destroy water says
Container is the operative word, because the definition of which is:
Per the rules creatures, nor parts of them that are still attached, are not objects (Object and Creature being mutually exclusive rules categories).
Because a creature nor any contiguous part of it is only an object, it cannot be a container, and therefore water in a creatures mouth (or any other part of its body) is not a valid target fort the spell.
(Note, this also applies for Destroy Water, which again specifies an open container)
The fact the spell lacks information for such an action would be very good supporting evidence for not being able to use the spell in such a fashion
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Even if you could do that, and there's strong evidence you can't, the creature wouldn't be incapacitated. It would just stop breathing. What happens next depends on your interpretation of whether having your lungs filled with water is "holding your breath" or "choking".
If it's "holding your breath", a creature can do that for a number of minutes equal to 1 + its CON mod, minimum 30 seconds. That's at least 5 rounds (longer than the average combat) to make sure the Druid who just filled its lungs with water has a bad day.
Here's the thing though: even if it's "choking", the creature still has a number of rounds equal to its CON mod (minimum 1 round) to get air before it falls unconscious. How long does it take to expel enough water from your lungs that you can breathe enough to stay conscious? I don't think there's specific rules for that, but "six seconds" isn't out of the question, particularly if someone helps you. So the creature loses an action coughing up water, and then is fully ready to go because there's no defined penalty for having a little bit of water in your lungs. Except now it's probably holding its breath (which it can do for at least 30 seconds), so that clever little trick won't work on it again.
In summary: this meme is pretty clearly forbidden by the rules of the spell, but even if it wasn't, the most you're likely to accomplish is burning one or two enemy actions to first aid. Not a particularly efficient use of a 1st level spell slot in my opinion.