Despite the overall bleak tone of the book overall, the ending is actually kinda-sorta happy: Lipsha has just bonded with his Daddy and come to terms with his parentage (after spending most of the book super angry at having been abandoned), and he's getting ready for a pleasant drive home in "June's" car:
I still had Grandma's hankie in my pocket. The sun flared. I'd heard that this river was the last of an ancient ocean, miles deep, that once had covered the Dakotas and solved all our problems. It was easy to still imagine us beneath them vast unreasonable waves, but the truth is we live on dry land. I got inside. The morning was clear. A good road led on. So there was nothing to do but cross the water and bring her home. (16.4.55)
Lipsha seems pretty jazzed to have the car (and not just because of the convenience of having transportation). King had purchased the car with the insurance he got from June's death, and so people sometimes refer to it as June's car. Now that Lipsha knows that he's June's son as well, it's probably a nice warm and fuzzy feeling to have "her" car.
Speaking of "her," by the way, that final line is a bit ambiguous. Ostensibly he's talking about the car itself, but it's entirely possible he's kind of thinking of the "her" whose money bought the car—and who died on her way home at the very beginning of the book while trying to walk on water—might somehow, symbolically, finally actually be crossing the water coming home.
After all, as we've already discussed elsewhere, June has totally been a Christ figure in this novel, and so it's really kind of like this moment is her resurrection—even if she's not literally alive, the fact that Lipsha finally feels a connection with her is what actually makes her present enough to get home.