In The Tempest, what does Gonzalo means when he says "the ship were no stronger than a nutshell"?

Gonzalo is referring to the ship's boatswain, the hardy, sea-faring soul who's just rudely told Sebastian and Antonio to get below deck and let him get on with his work. This is clearly one tough sailor, and though Sebastian and Antonio curse him to his face, Gonzalo has a kind of sneaking admiration for him.
In a fit of rage, Antonio tells the boatswain that they're less afraid of drowning than he is. That's when Gonzalo says that the boatswain will never drown, even if the ship were no stronger than a nutshell.
If you think about a nutshell, you'll realize how incredibly fragile it is, how easy it is to break. So what Gonzalo is saying here is that the boatswain is so fearless and strong that he wouldn't drown even if his ship were as fragile as a nutshell and started to break up.