The speaker looks back over a lifetime of imagining an ocean spilling across the road outside his house. As a young man he sits and watches the waves from a lawn chair. Later he stands, hears seagulls and tourists, and lets the pretend water run over his toes. Each stage brings him closer to the sea, until he is no longer at its edge but at its bottom.
That last image changes the mood. The "rough cement" under his feet reminds us the ocean is not real, yet he sits calmly with his arms on his crossed legs, waiting to see whether his lungs will fill with "harmless water." The word "harmless" is odd and a little unsettling. It could suggest peace, a wish to be fully immersed in imagination, or a quiet acceptance of aging and an ending. The poem never says which, and the calm tone makes it feel both dreamy and melancholy.