In Tuck Everlasting by Natalie Babbitt, how is the movement of the pond's water meaningful to Winnie and Tuck's conversation about the life cycle?...

Angus Tuck uses the movement of the pond's water as a metaphor for the circle of life.  

The Tuck family is immortal.  They are stuck in their present age and physical bodies.  They don't age and change with time anymore.  Angus Tuck takes Winnie out onto the pond in Chapter Twelve.  He does this in order to have some alone time with her and explain why she must keep the spring a secret.  

Angus begins the entire conversation with Winnie by explaining to her that even though the pond's water looks stagnant and unchanging, it is actually changing all of the time.  It flows into the pond from one end, and it flows out of the pond at 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 always new and moving on part is important for Angus, because he and his family can no longer do that.  They are stuck.  Angus goes on to further explain the entire water cycle to Winnie.  He doesn't say evaporation, condensation, and precipitation, but he does tell Winnie that the pond water eventually ends up in the ocean.  From there it will be transported via clouds to some other location and rained down.  

"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."

Angus explains to Winnie that the entire thing is a cyclical "wheel."  He explains that an always turning and changing cycle is normal.  That's how life works.  He stresses that he wishes that he could get back on the wheel again.  He stresses it so much that he tells Winnie that the ability to die again would be the greatest thing in the world.  Angus explains that without dying he's not really living.  He just exists.  

"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."