Realistic, Bleak, Hopeful
Even though the genre is sci-fi, the tone is pretty realistic. Nailer knows that his pipe dreams of working a clipper ship are just that—dreams that most likely won't be realized. And the language reflects the gritty drama that feels far truer than Disney's Cinderella. Just look at this passage:
Nailer had felt his belly up against his spine enough times that he wondered what it would be like to have so much food. Wondered how it would feel to never wake in the middle night with his teeth chewing on his lips, fooling himself into thinking that he was about to eat meat. But it was a stupid fantasy. (1.12)
Hunger is real. So is hard work. Nailer almost dies working light crew to strip ships of their usable parts, and if Nailer hadn't found the clipper and rescued Nita, we know that his options would be incredibly limited. Plus Bacigalupi's society seems like only a slight exaggeration of how our world currently works—without help from the outside, without a little fortune, it's hard to rise above the life we're born into.
Much of the content is also bleak. The Bright Sands Beach might be sunny, but the world has become much darker. Sloth might have to sell parts of herself to survive; Nailer's father wants to kill him; corporations fight one another for control of millions of dollars; and they have to contend with pirates in the north seas. Poverty is rampant both on the beach and in Orleans, and even the natural world seems sinister. Check out this description of the city under the ocean:
They sped above the mossy broke-back structures of a dead city. A whole waterlogged world of optimism, torn down by the patient work of changing nature. (16.53)
More than the destruction described, the word patient is what really makes this into a bleak description. Patience implies steadfastness, a willingness to wait, to try again and again to realize the devastating goal of consuming an entire city. Despite human belief that technological advancements can make the world a better place, nature has won. It's not the most cheerful backdrop for a tale of high-seas adventure and heroism.
But there is hope. It's not much, but it's there. The hope appears when Nailer chooses to do good: When he saves Nita from the clipper ship and from his father, when he throws his lot in with Captain Candless to search for Nita, and when he gets Sadna and Pima out of working as ship breakers forever. It's not much, and Nailer definitely has to contend with an incredible amount of adversarial people and positions, but there's enough hope to make a difference in his attitude and his actions.