What risks at sea does Shylock enumerate to Bassanio?

In act 1, scene 3, of the Merchant of Venice, Bassanio asks Shylock for a loan of 3,000 ducats on Antonio's credit. Shylock provides Bassanio with a risk analysis of this loan. He knows that Antonio is wealthy but that most of his wealth is tied up in the risky business of transoceanic shipping ventures. Shipping has its risks, which Shylock thoroughly lays out to Bassanio.
First of all, Shylock mentions that there are pirates, rats as he calls them, both on land and at sea. Piracy was a real concern in the sixteenth century. With vast fortunes moving around by ship, particularly ships returning from the New World, piracy was a huge industry. Entire fortunes could be lost in a single pirate raid.
Shylock also mentions the natural dangers of "waters, winds, and rocks." It was not uncommon for ships to get lost at sea. In the days before reliable navigation and weather forecasting, storms, doldrums, and navigational errors frequently spelled disaster for ships, their crew, and their cargo.
Shylock is making it clear that he feels loaning money to Antonio has its risks. Despite the risks, he is still willing to lend Antonio the 3,000 ducats that Bassanio requested, but he would like to meet with Antonio first and make special conditions.