Best way to decide is this : should a cantrip or level 1 spell one shot a target ?
No. No it shouldnt. Disintegrate can because its a very high level spell slot. But resources wise, foot the fucking bill please.
But I would allow its use to torture a captured target by waterboarding.
Well it depends on the circumstance. The "captured target" you mention is helpless. You absolutely can oneshot him with any number of cantrips, or other mundane actions. You could oneshot him with a "use an object" action if that object is a lever controlling a trapdoor over a deep pit.
Point is, you certainly can drown someone with 30 gallons of water. You just have to set up for it correctly. I don't really like the implication of your comment and the original post that it's unreasonable to try such a thing just because the number used to categorize the spell is too low.
If the target is already captured or subdued, nothing can oneshot them. That's just coup de grace.
"one shot" says to me that it instantly kill or removes as a threat an as-of-yet untouched and un-interacted-with target.
When people try to drown someone with create water they aren't talking about creating water then drowning the person the old fashioned way, it's "I cast create water in the lungs of that guy!"
Other popular "ideas" include - -Casting light on someone's eyes so they go blind -Trying to target eardrums with shatter -Conflating charm person with dominate person -Attacking with mage hand -prestidigitation solves every problem and has no limits
It's not that there is an arbitrary "number too low" problem, it's that these spells explicitly state what they can do. Players sometimes feel "creativity" means they perform actions the spell doesn't allow, and moreover are actually achieved by much more powerful spells.
it's "I cast create water in the lungs of that guy!"
I mean that just obviously doesn't work because you don't have a clear path to that guy's lungs. His clothes and skin and bones are in the way. Furthermore, a creature's lungs are not a container, and even if they were, it would be difficult to argue that they are an open container. But there are still a wide variety of ways it would be possible to drown someone using create water, and personally, I would sooner assume that the hypothetical player involved is considering one of those ways rather than a completely nonsensical way that doesn't even begin to fit with the basic rules of spellcasting.
https://www.reddit.com/r/dndnext/comments/cemh20/using_createdestroy_water_to_kill_someone/
https://www.reddit.com/r/DnD/comments/vqyu78/create_or_destroy_water_in_someones_lungs/
https://www.reddit.com/r/DnD/comments/cdtb5c/can_create_food_and_water_be_used_to_drown/
https://www.reddit.com/r/DnD/comments/ql764n/so_weve_all_heard_of_the_fill_their_lungs_with/
https://www.dndbeyond.com/forums/dungeons-dragons-discussion/tips-tactics/31598-flooding-somones-lungs
https://www.quora.com/Can-you-create-or-destroy-water-to-kill-someone-in-D-D
There are more. So many more. I just got tired of copying and pasting.
Casting light on someone's eyes so they go blind
Funny enough, in b/x D&D this was explicitly allowed. They got a save though, and light was a 1st level spell, not a cantrip so it was more like color spray that also gave you a magical light that followed you around afterward than an infinitely castable save or suck cantrip.
Its a resource management game. Either you use your imagination and roll a few chance dice, or you use an appropriate resource for it. Which is why killing a target that isnt already captures with a cantrip one shot doesnt work.
The test I have is if my players would call BS on an NPC doing that to them.
Drowning a PC a turn using a cantrip? That's BS and every player knows it.
I allow for creativity, but doing something cool without resources requires to be lucky. Either you use resources (abilities, spell slots, items, weapons, terrain, etc) or you hope to get lucky on a low chance plan.
I'd simply argue that lungs are too convoluted in their layout to be considered a container. Mouth is theoretically not an open container because it has multiple openings.
Just gently remind the player that it is a game, if they insist tell them that a Chimpanzee has a STR of 14 and since most fighters would have that or well above instead of doing the ole 'have at thee' they'd could just grapple them and crush their windpipe, if you have some ogre they realistically would just grab the wizard and pop them like a waterballoon but obviously no one does this because it's boring and makes for a bad game.
I think that sounds really interesting, but for a different kind of game from what many people would want to play.
