The poem is built around a strange, quiet question: should the ground simply step aside and let a sea take its place? The title, "Flood control," suggests we're meant to think about how people manage water, but the speaker flips the usual approach. Instead of building walls or barriers against the water, the speaker imagines the earth itself giving way.
The second stanza is the oddest and most interesting part. The sea is called "poor and still," which makes it sound humble, almost pitiable, rather than threatening. The speaker then suggests the ground ought to hold back from our "hydrostatics," a technical word for the pressure of water at rest, and become a bed. That image turns the land into something soft and welcoming, a place where water can lie down.
The mood is calm, a little playful, and gently strange. The speaker seems less afraid of water than willing to accommodate it, as if comfort might work better than control.