The poem is only three short lines, but it holds a quiet leap of imagination. Someone lies in bed listening to rain on the roof, and the sound seems to carry them somewhere else. The "broad water belonging to ships" is the open sea, a place of distance and travel, and the speaker says "we" want it. That small word makes the longing feel shared, something ordinary and human rather than private.
The title, "Roof music," helps us hear the scene. The rain becomes a kind of song, and the poem suggests that its steady drumming brings the sea to mind. Judging by the wording, the speaker doesn't really have the ocean, only its echo, which is why the longing feels gentle rather than sharp.
The mood is drowsy, calm, and a little wistful. The ending, "bedtime rain," settles everything back into a cozy, safe moment: wide horizons imagined from the comfort of a bed.