The sea here used to look
As if many convicts had built it,

Standing deep in their ankle chains,
Ankle-deep in the water, to smite

The land and break it down to salt.
I was in this bog as a child
When they were all working all day
To drive the pilings down.
In this first image of man vs. nature, the speaker sets up how humans are essentially slaves to society and are almost seen as prisoners when performing the mundane and futile task of taming the land (nature) in order to build something artificial on top of it.
At Darien Bridge