Antonio uses a metaphor, a comparison not using the words like or as, when he likens his sadness to an object or creature, stating,

how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born.

This shows he feels divorced from his sadness and cannot fully understand it.

Salerio responds with a metaphor when he compares Antonio's state of mind to a ship "tossing on the ocean." Salanio, another friend, then uses visual imagery, describing something we can see, along with alliteration, when he says,

Peering in maps for ports and piers and roads.

We can imagine Antonio worrying about his ships, poring over maps (although, in fact, he is not worried about his ships). The alliterative repetition of the p sound at beginning of peering, ports, and piers brings added emphasis to those words. Salanio's statement above also uses polysyndeton, which is when a series of conjunctions are repeated. Normally, a person would say "ports, piers, and roads," but Salanio adds an extra "and," bringing emphasis to each of his three terms and adding a sense of rhythm.

Salerio then uses an image-rich metaphor, comparing time to the sands in an hourglass, as well as adding alliterative and consonant s sounds, when he says he would

see the sandy hour-glass run. (emphasis added)

When Salerio speaks of "two-headed Janus," he is using an allusion. He makes another time-based reference, as Janus, for whom the month of January is named, looks both forward and backward.

Bassanio uses a simile when he compares Portia's hair to a golden fleece. This is also a double-entendre, in that Portia's gold refers to both the color of her hair and her wealth, both of which Bassanio desires. It is also an allusion to Jason and the argonauts: obtaining the fleece means that Jason can claim his rightful place of authority, just as marrying Portia will give Bassanio status and authority.

The first scene of the play abounds in similes and metaphors, and personification and allusion are used as well.

Similes include “laugh like parrots at a bagpiper,” and Portia’s “sunny locks / Hang on her temples like a golden fleece…”; the latter is also an allusion to the prize that Jason pursued in ancient Greece. An extended instance is Bassanio’s description of the “infinite deal of nothing” that Gratiano speaks:

His reasons are as two

grains of wheat hid in two bushels of chaff: you

shall seek all day ere you find them, and when you

have them, they are not worth the search.

In a metaphor involving sea travel and the merchant fleets, Salarino calls the fleets “pageants of the sea.” Gratiano, speaking of affection, teases Antonio about trying to get him to say nice things so he won’t feel sad: “But fish not, with this melancholy bait,/ For this fool gudgeon, this opinion.”

Extended metaphors include Bassanio’s description of his intended approach to reversing his fortunes through Antonio’s lending him more money. He provides an example of literally shooting a second arrow in the same direction to find a lost one, then says

…if you please

To shoot another arrow that self way

Which you did shoot the first, I do not doubt,

As I will watch the aim, or to find both

Or bring your latter hazard back again….

Gratiano personifies an emotion, love, in describing his affection for Antonio: “it is my love that speaks.” The characters personify various inanimate objects, including the ships. Referring to a grounded ship, Salarino says it would “kiss her burial.”

Other allusions, especially to Roman gods (as the characters are Italian), include reference to “two-headed Janus” in describing people with very different temperaments.

Figures of speech are tools that authors use to help readers, or in this case, audience members, identify with real world situations, objects, or senses in order to relate to the what the characters are experiencing. In the first act and scene of Shakespeare's The Merchant of Venice, there are many metaphors, idioms, similes and allusions to find. A few of the many figures of speech that can be found in the first act are shown below.  

In the opening scene of the play, Antonio tells his friends that he is sad, but he can't figure out the reason why. Salerio provides his friend with a metaphor to show how his sadness forces him to suffer: "Your mind is tossing on the ocean" (I.i.8). Salerio then extends this metaphor for his friend, but also to let the audience know that Antonio has a lot riding on the ocean by way of merchant ships, literally. Since all of Antonio's money is invested on his ships currently at sea, he is probably more anxious about their success than he is sad. 

Next, Solanio agrees with Salerio by saying that if he had so much invested on ships at sea, he would also be worried. To illustrate to Antonio what anxiety feels like to him, Solanio uses an idiom:

"I should be still

Plucking the grass to know where sits the wind

Peering in maps for ports and piers and roads, 

And every object that might make me fear

Misfortune to my ventures out of doubt

Would make me sad" (I.i.17-22).

Salerio then uses a comparison between his breath cooling his soup and the winds at sea that might threaten his ships, if he had any:

"My wind cooling my broth

Would blow me to an ague when I thought

What harm a wind too great might do at sea" (I.i.22-24).

When Antonio tells his friends he isn't worried about his merchandise at sea, and that he really doesn't know why he is sad, Solanio says he just isn't happy. Then he uses a simile and an allusion to show that Antonio simply isn't laughing, which can make one sad:

"Some that will evermore peep through their eyes

And laugh like parrots at a bagpiper

And other such vinegar aspect

That they'll not show their teeth in a way of smile

Though Nestor swear the jest be laughable" (I.i.52-56).

Solanio explains that some people "laugh like parrots," while others may refuse to smile even if someone great like Nestor tells them a joke is funny. With this simile and allusion to Nestor (a great Greek hero from The Iliad) Solanio means to say that happiness can be a personality trait. Someone simply is or isn't happy sometimes and it is unexplainable.