Why is Antonio unconcerned about his ships at sea in The Merchant of Venice?

Antonio is a wealthy merchant with several ships out to sea. While he is cash poor at the moment, he is certain that at least one of his ships will make it back to Venice. At that point, he can easily take care of his loan to Shylock.
Bassanio is concerned at the terms of the loan agreement Antonio has made with Shylock, which allows Shylock a pound of Antonio's flesh if he should fail to repay the loan. Antonio, however, pooh-poohs these worries. He says to Bassanio:


Within these two months—that’s a month before
This bond expires—I do expect return
Of thrice three times the value of this bond.


Antonio says he has a wide margin of time and several opportunities for the money to come in before the loan is due.

Part of Antonio's confidence may arise from the fact that he has always been successful and can't really imagine the kind of disaster that might befall him.