Is the sentence went into sea grammatically correct?

The sentence "Went into sea" is not grammatically correct. It's missing an article before "sea". Here are some ways to correct it:

* Went into the sea: This is the most common and grammatically correct way to say it.

* Went into a sea: This implies going into a specific, smaller body of water within a larger sea.

The choice between "the" and "a" depends on the context and what you are trying to convey.