We first hear the rumor that Antonio has lost one of his ships in the opening scene of act 3. Solanio and Salarino enter the scene discussing the news that one of Antonio's ships, carrying valuable cargo, has wrecked off the English coast. Of course, when Shylock arrives and hears the news he is overjoyed at Antonio's misfortune; now he might have his chance at revenge.
In the following scene, we learn that the situation is even worse for Antonio. Bassanio receives a letter from Antonio informing him that all his ventures have been lost at sea. Of course, since Antonio had borrowed three thousand ducats from Shylock—with his ships as collateral—he would now owe the moneylender a pound of his flesh, as these were the terms of their agreement.
The characters in The Merchant of Venice are acutely aware of the risks involved in shipping. We hear just how dangerous an investment shipping is from Shylock in act 1, scene 3 when he enumerates all the risks to Bassanio, ranging from pirate raids to storms at sea. It is almost as if Shylock expects Antonio to suffer the loss of all his investments.