The book ends with Tuna, but let's back up a moment first: She is able to stand up to her father now that his arms are tied, and she stuffs a nasty sock into his mouth after he says something totally insane about how Derek is a maniac for trying to drown him. Um, who is the maniac here?
Anyway, help arrives and Wahoo flies with the two wounded men. Derek is totally glowing from his bravery and immediately resorts to acting like he used to by asking if he was videotaped while taking Jared down (so his brief character growth shrivels once again).
Here is where the meat of the ending is: Tuna watches while her father is arrested—a perfect end to the chaotic day. And, in case you're not convinced that Tuna is just as stoked as the rest of us to see her dad in cuffs, check this out:
He spit out the sock and said, "I want a lawyer."
"You got a name, mister?" asked one of the policemen.
"No comment."
"Homo Sapiens," said Tuna, "but a really lame specimen."
She tossed the torch into the shallows, where it hissed and went cold. (26.49-53)
In these lines, we can see that—in addition to Tuna relishing her dad's punishment—there is a focus on humans here. There is a range of human potential, and Jared happens to be at the intensely lame end of the spectrum. On the other end is Wahoo, pure of heart through and through. For her part, Tuna gets the last word because she sets up the contrast between good and evil.