The poem opens with a small, wry observation: the sea is wonderful to be near, but it won't amuse you. It keeps you company without putting on a show. That tone of dry honesty shifts quickly into something heavier. "We go out / all in mourning" suggests that people approach the water already carrying grief, or at least a solemn mood.
The last line holds the poem's key idea. The speaker says we believe the sea is "a sermon and a pall." A sermon lectures us about life and death, and a pall is the cloth draped over a coffin. So we treat the sea as both a preacher and a funeral shroud. The word "believing" matters, though. It hints that we may be projecting this meaning onto something that is simply there, vast and indifferent.
The mood is quiet, sombre and a little wry. The title's "wine-dark sea" adds an old, ancient weight, as if people have always looked at the water this way.