We don't know if you've noticed, but the ocean is a pretty potent symbol in film and literature.

You see the Big Blue pop up in all sorts of canonical works. The Old Man and the Sea. The Rime of the Ancient Mariner. Waterworld. (Ten points to Gryffindor if you can tell us which of those texts is not considered a groundbreaking work of art.)

It's totally massive, which means it's a great symbol of everything that's overwhelming or unknowable. Just look at someone Brock Lovett, who has the best technology available for exploring the ocean—even he can't find everything he's looking for in the deep waters around Titanic.

Of course, that's largely because Rose has been secretly holding onto the diamond Lovett is looking for, but the point is that the ocean is just too freaking deep and large to understand completely.

For this reason, Rose compares its watery depths to a woman's heart:

ROSE: […] And I've never spoken of [Jack] until now. Not to anyone. Not even your grandfather. A woman's heart is a deep ocean of secrets.

To paraphrase Mean Girls, that's why the ocean is so big: it's full of secrets.

Perhaps that's why she sends the diamond back into the water at the end—it symbolizes her love for Jack, which she kept private for most of her life, and so she wants to return it to that same obscurity.

Pretty crummy for Brock, but it makes a certain amount of sense. The ocean, like life (including the heart), is not ever really supposed to be known, possessed, or understood completely.