Learn English – Went to or went into in a sentence

First, you have to write "the sea". Also, this is not an issue of grammar but meaning.

More context is necessary to choose, but in general:

He went to the sea alone.

This means you went to the area near the sea. If I went to a coastal area, I could say this. But it does not really mean in/on the water.

He went into the sea alone.

This means he went from being "out of the water" to "in the water".

He went in the sea alone.

Same as "into" when using went, since motion is indicated.

Generally I would use on the sea when you are traveling in a boat.

Both are correct but used in different places. It depends on which you would use aside from the introduction. For example, if I said;

I'm going to learn about maps.

Then I would use introduction to.

I'm going to be introduced to maps.

explanation of introduced to

Also when you use this, it generally means you are being introduced to someone not something.

Now if I were saying;

I'm going into a seminar.

Then I would use introduction into.

I'm being introduced into a seminar.

explanation of introduced into