Why is Antonio sad at the very start? His ships are ruined He needs to marry He is worried about best friend Bassanio His estate is at loss

I think you are correct,

In sooth, I know not why I am so sad.
It wearies me; you say it wearies you.
But how I caught it, found it, or came by it,
What stuff ’tis made of, whereof it is born,
I am to learn.
And such a want-wit sadness makes of me,
That I have much ado to know myself.

We can, however read into this further. Antonio is concerned about his friend Bassanio's melancholy. I might go with that choice on your list.