You will want to look in chapter 12 for textual evidence regarding this question. The answer is largely explained by Tuck himself because he uses the pond and boat as a metaphor to explain the cycle of life to Winnie Foster.
Remember, the Tuck family is immortal. Not only that, they do not physically age anymore. They don't change with time, and Winnie has started to think that could be a positive; however, Angus takes Winnie out onto the pond in order to better explain how his family's immortality isn't really living.
Angus starts by explaining to Winnie that the pond's water looks stagnant and unchanging; however, the water is constantly changing. It flows in on one end and flows out the other end.
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.
The moving on is critical for Angus. It is critical to him because it is something that he and his family can no longer do. He continues to elaborate on the water movement and explains to Winnie that the pond's water will eventually reach the ocean, where it will be taken up into clouds to be rained down somewhere else.
"Know what happens then?" said Tuck. "To the water? The sun sucks some of it up right out of the ocean and carries it back in clouds, and then it rains, and the rain falls into the stream, and the stream keeps moving on, taking it all back again."
Most readers should recognize that Angus just described the water cycle, and that is often diagrammed as a circle. To Angus, it is cyclical "wheel." An always turning and changing wheel is how life works. It's normal. Unfortunately, nothing about his life resembles the wheel anymore, and that greatly saddens Angus. He deeply desires to get back on the wheel. He wants to get back on the wheel and have the ability to die because life without death isn't really living. He is simply existing like a non-living rock.
If I knowed how to climb back on the wheel, I'd do it in a minute. You can't have living without dying. So you can't call it living, what we got. We just are, we just be, like rocks beside the road.