In Act I, Scene iii of The Merchant of Venice, explain Shylock's words, " Ho, no, no, no, no; my meaning in saying he is a good man is to have you understand me that he is sufficient. Yet his means are in supposition: he hath an argosy bound to Tripolis, another to the Indies; I understand, moreover, upon Rialto, he hath .....I think I may take his bond."
In The Merchant of Venice, Antonio would do anything to help his friend, Bassanio who wants to pursue Portia's hand in marriage but has "disabled mine estate"(I.i.123). He has squandered all his own wealth so Antonio encourages him to "try what my credit can in Venice do" (180) as even Antonio's "fortunes are at sea" (177) so he does not have the funds to lend to his dear friend.
Shylock and Antonio are old enemies and, as the audience will learn shortly, Shylock hates "him for he is a Christian." (I.iii 37). Shylock is interested in Bassanio's proposition of a loan of three thousands ducats. He knows that Antonio is a man of means and "is sufficient." Shylock also knows that Antonio's "means are in supposition" and tied up abroad, en route between Venice and Tripolis, Mexico, England and the Indies. In considering Bassanio's request, Shylock is basically thinking out loud and reminding Bassanio and the audience that ships are not one hundred percent reliable, sailors are only human and pirates abound "in the peril of the waters."
Having contemplated this, Shylock knows Antonio is good for "his bond" but the audience can already imagine Shylock's scheming to get the better of Antonio; such as will be revealed later.