As a general statement, which one is more correct?
The pool looked gorgeous in the night.
The pool looked gorgeous at night.
The pool looked gorgeous during the night.
All three could be grammatically correct in colloquial English depending on the context. Here are some examples, though not exclusive:
The pool looked gorgeous in the night, but in the morning we saw trash at the bottom.
The pool looked gorgeous at night. [This is fine as is, and is the most likely usage.]
The pool looked gorgeous during the night, while lit by the floodlights.
Example 1 implies it might not always look good, and 3 implies a duration. English is flexible, and in this case, there is no intrinsically wrong answer.
Looked gorgeous in the night.... poetic/literary/slightly "heightened" diction.
At night.... normal register
During the night... typically, things happen during the night; things do not seem beautiful during the night. The word "throughout" would probably be a better choice than "during" if you want to link it to looking beautiful.