Why is Gonzalo confident that the boatswain will not die by drowning in The Tempest?

Gonzalo shows he is a shrewd judge of character, as he more than once expresses confidence the boatswain won't let them all drown during the violent storm that opens The Tempest. He perceives that the boatswain is a tough, defiant person who will do whatever it takes to survive. He is competent and not a person who will give into fear.
While the passengers are panicking, Gonzalo tries to point out the toughness of the sailor they are depending on, saying he is more likely the type to be hanged than to give into a storm. He also says is sure that this boatswain wouldn't let them drown even it the "ship were no stronger than a nutshell."
Gonzalo is right. The boatswain does what it takes, which includes not being afraid to rudely tell the high-ranking guests to be quiet and stay out of the way. Of course, he can't overcome Ariel's magical powers, but he fights the good fight. The scene also helps characterize Gonzalo as having more sense that his royal patrons.

In the opening scene of the play, the royal court is aboard a ship that is being viciously tossed by powerful waves in the middle of an intense tempest. As the Boatswain shouts orders to the sailors, Alonso and Gonzalo attempt to give him directives and he replies in an insolent manner. The Boatswain directs the royal court to get below deck and mentions that their titles mean nothing to the storm. Gonzalo is offended at the Boatswain's lack of respect and proceeds to say,

"Methinks he hath no drowning mark upon him. His complexion is perfect gallows. Stand fast, good Fate, to his hanging. Make the rope of his destiny our cable, for our own doth little advantage" (Shakespeare, 1.1.23-27).

Essentially, Gonzalo believes that the Boatswain is such a despicable person that he is destined to be hanged instead of drowning at sea. In Gonzalo's opinion, the Boatswain acts more like a criminal than an integral member of the ship's crew. The Boatswain continues to curse the royal court and speaks disrespectfully towards Sebastian and Antonio, which prompts Gonzalo to reiterate his prophecy that the Boatswain is destined to be hanged rather than drown at sea.