The speaker spends a day simply sitting by the sea, and the poem is a quiet attempt to say what that feels like. The sea isn't described with waves or colors. Instead it's called "that sudden country," as if you could walk up to the shore and find yourself at the edge of a foreign land, one that appears all at once and has no borders you recognize.
The second part is stranger and more interesting. The sea's "business," its main job, is "absence." It is a huge space where things aren't: no buildings, no people, no landmarks. Yet the poem calls this absence "home to prayers." Emptiness, it suggests, is where people send their hopes and wishes, perhaps because there's nothing there to interrupt them.
The mood is still, reflective, and a little awed. The speaker seems calm rather than lonely, treating the sea's blankness as something spacious and almost sacred. The short lines slow you down, much like sitting and watching the water.