This one won't take much brain power—Tuck does most of the work for us. Out on the pond, Tuck lays it all out:
"Know what that is, all around us, Winnie? […] Life. Moving, growing, changing, never the same two minutes together. This water, you look out at it every morning, and it looks the same, but it ain't. All night long it's been moving, coming in through the stream back there to the west, slipping out through the stream down east here, always quiet, always new, moving on." (12.4)
The water of the pond represents life; and life is always moving on. Not so for the Tucks. And Tuck uses the rowboat to describe their not-like-water state:
"[This water] goes on," Tuck repeated, "to the ocean. But this rowboat now, it's stuck. If we didn't move it out ourself, it would stay here forever, trying to get loose, but stuck. That's what us Tucks are, Winnie. Stuck so's we can't move on. We ain't part of the wheel no more. Dropped off, Winnie. Left behind. And everywhere around us, things is moving and growing and changing. You, for instance. A child now, but someday a woman. And after that, moving on to make room for the new children." (12.8)
The Tucks are permanently stuck, while everything around them—Winnie included—continues to move. And guess what? Tuck isn't thrilled about it. Since he can't change his own fate, he wants to help Winnie choose hers. He wants her to live.