While the film's title is often used without an article as a proper noun, if you are referring to it in a sentence (e.g., "The movie Mar Adentro "), the correct article to use is: Mar adentro Quick Grammar Facts Gender: Mar (sea) is generally masculine in Spanish.
In Spanish, the grammatical article for the title is the masculine singular el . Mar adentro
You use el because the word mar is the subject of the title. While the film's title is often used without