What does the line "Your mind is tossing on the ocean" from Shakespeare's The Merchant of Venice mean?

The words are spoken by Salarino in the opening scene of the play. He can tell from the look on his face that something's eating his good friend Antonio. Salarino uses the metaphor of a ship being tossed about on stormy seas to describe his friend's troubled mental state. That's what he means when he says "Your mind is tossing on the ocean." Just imagine, for one moment, a ship caught in the middle of a particularly violent storm. It would be heaving up and down, just about ready to break up and sink at any moment. That's what Antonio's mind is like as the play begins.
The maritime metaphor is particularly appropriate as Antonio, who's a merchant, has all his money tied up in his ships' valuable cargo. If any one of his ships should sink in stormy seas, then he'll be in serious financial trouble. Indeed, that's precisely why Salarino thinks that Antonio looks so troubled; he's worried about his fleet of merchant ships.

This line appears in the opening of the play, at the beginning of the first act. Antonio is wondering aloud to his friend Salarino why he feels so sad. Antonio says it wearies both of them for him to be so depressed and comments that he doesn't understand himself.
Salarino then says to Antonio, "Your mind is tossing on the ocean." By this he means that Antonio is sad because he is constantly thinking and worrying about the ships he has out at sea. In other words, Salarino is stating that Antonio is stressed and preoccupied with worry that his ships won't come home and his business ventures will fail. Another friend, Solanio, concurs, telling Antonio that he would be in a constant state of worry if he were involved in such risky business.
However, Antonio says that's not why he's sad. He notes he's diversified his risk between different ships and isn't particularly worried. When his friends then suggest that he must be sad because he's in love, he denies that, too, but given that his close friend Bassanio wants to marry Portia, we might suspect that love and fear of losing Bassanio's companionship might be the cause of his depression.