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Он отправился в море, и наловил там много рыбы.

The above sentence is from my essay, and my teacher marked the comma as a punctuation mistake of mine. I strongly disagree with him. Being unable to persuade him that I am right, I decided to humbly ask native speakers on this SE. Please kindly resolve our argument.

I put this comma in order to convert a simple sentence to a complex sentence with two independent clauses, the second one having zero subject. I knew that the sentence would be perfectly okay without the comma, but I wanted to make a pause in this sentence. I wanted the reader to first imagine the setting off for fishing, then make a pause, and then imagine the fishing itself. Without the comma the sentence would read very quickly and be a single idea. I even considered putting a full stop instead of the comma.

At any rate, I believe that putting this comma is not against the Russian grammar rules. I believe I have the right to convert a simple sentence to a complex sentence by putting a comma. I believe that an independent clause with zero subject is still a valid independent clause. I believe that the phrase "и наловил там много рыбы" would even make a valid isolated sentence.

However, my teacher insists that putting this comma is a mistake. He says that a sentence with one subject and two verbs separated by the conjunction и must always be written without a comma.

I explained him why I had put this comma, and he laughed and said that only great Russian writers have the privilege of putting commas for reasons like this. He says that if I were Pushkin, he would consider my comma as a punctuation sign with its own deep meaning. He went as far as saying that my comma is a mistake because I am just a student. But I strongly believe that whether a comma is a mistake cannot depend on the author.

My question is this: Who is wrong - I or the teacher? In other words, is it a mistake to put this comma?

It is a simple sentence with two homogeneous predicates. There's only one subject both predicates refer to. That's why your teacher is right, there should be no comma in this sentence.

It is a common mistake nowadays when people are non-educated enough to put commas whenever they take breath. Even breaking the punctuation rules and common sense. That's why it sooner irritates than makes us pause.

I just want to add to the previous answer that putting a comma doesn't convert a simple sentence to a complex sentence. There is no such a rule and your teacher was totally right telling you that it's only writers' privilege to make one more simple sentence out of nothing, just by putting a comma.

A comma doesn't create any more main clauses than there are in the sentence. But if you don't put a comma you just make a mistake: you can't see the right ammount of grammar structures - main clauses.

I advice you to identify or underline all the main clauses in the sentence while you study. This is extremely useful. And after all remember that russian native speakers make really a lot of mistakes putting commas and mostly we take it easy. We really like to put commas where we feel to :)

I disagree with placing a comma there even if it were author's intention, but there is an alternative way without restructuring the sentence.

The pause in your sentence is implicit and well-understood: actions отправиться на море and наловить много рыбы are consecutive, and the latter takes some time to complete. The question is, what do you want to signify with the pause?

And as long as you provide some context that explains the pause, you could use a dash in place of a comma:

Он отправился в море - и наловил там много рыбы.

The context is important though: for example, previous sentences could tell about unsuccessful attempts to catch fish on a river.

Он провёл утро с удочкой на берегу реки, но не поймал ничего. Он отправился в море - и наловил там много рыбы.

This would look perfectly fine.

In my strong opinion it's you as the author who makes this call, not your teacher being in this case in a capacity of a passive reader.

I totally can get your intent and would read the sentence just as you've punctuated it.

If you were doing a punctuation exercise of some sort, then the comma could have been considered a mistake. And also if you were an inexperienced and ill informed fresh student of Russian.

If it's a piece of writing, an essay by a person who knows the rules and thus can bend them at will, then they're the one who decides.

One way to avoid such problem and to satisfy your teacher is to simply split the sentence into two as you yourself intimated. The effect of a pause you attempted to achieve with the comma will be maintained.