Is 海は青くないです less correct than 海は青ではない? I am just starting to learn and someone said my basic sentence was not polite. I'm trying to practice conjugating adjectives. My goal was to say ”The ocean is not blue".
海は青いくないです is ungrammatical. The adjective form of 'blue' is 青い. As I'm sure you've learnt, the negative form is made by removing the final い and adding くない. So it should be 海は青くないです. This sentence is polite. It is made polite by adding です after the i-adjective.
海は青ではない is grammatically correct but it is in plain form (i.e not polite). To turn this into the polite form ではない should be replaced with ではありません. Note that in this sentence the noun, 青, is used rather than the adjective, 青い, of the previous sentence.
For the adjective case you could also say 海は青くありません which is also polite but sounds rather stiff.
Since you are a beginner I should point out that 'not polite' doesn't mean rude. It just means a form you would use with someone with whom you are close.
Edit: Regarding the choice of whether to use a noun or adjective, please see this post. This makes the decision of the HiNative poster to change your sentence from 海は青くないです to 海は青ではない even more bizarre.
Just adding some thoughts: I guess it would be correct 青ではない if you are using it as a noun. Example:
好きなのは青ではなくて赤です [The thing I like is not blue, it is red]
But to me, just like the other person said, it also sounds off to say 海は青ではない。
Perhaps you could say 海は青色ではない. I am positive that saying it with 色 makes it acceptable.