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.
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.