I think Act I scene i is key in establishing both the setting but also many key themes that dominate the play and are used by Shakespeare as major plot devices. Of course, the very title, The Merchant of Venice, reveals the setting to us. Venice at the time the play was set was an incredibly important port, where ships belonging to merchants would sail off to gain luxuries from the New World and return, making perilous journeys, bringing wealth or ruin to their owners. This is established in this very first scene of the play, when Antonio describes himself as being in a very melancholic mood. His two friends, Salerio and Solanio suggest that his mind is perhaps focussed on his ships that have just left Venice:
Your mind is tossing on the ocean,
There where your argosies with portly sail,
Like signors and rich burghers on teh flood,
Or as it were the pageants of the sea,
Do overpeer the petty traffickers
That curtsy to them, do them reverence
As they fly by them with their woven wings.
We are thus introduced to a make or break world, with incredible risks but also incredible opportunities. It is important to note how Shakespeare weaves the setting into the plot - it is the report that Antonio's ships have been destroyed that puts him in such a perilous position with his pledge to Shylock, but also the news of the safe return of one of his ships resurrects him and gives him back his wealth and standing.