The title tells us the ocean is dying, but the poem itself opens in a surprisingly gentle tone. The sea is called "a most pleasant ark," a safe, welcoming vessel, something that carries life and shelters it. The speaker notes that it "did not seem ill," whether "there or here," as if looking at the water in different places and finding the same untroubled surface.
The final image is the sharpest. The sea is "like a king that knew / nothing of the world." A king is powerful and seems untouchable, yet this one is oblivious, perhaps unaware of his own decline, or of what is happening beyond his realm. The comparison suggests that the ocean's size and apparent calm can fool us, and maybe the ocean itself.
The mood is quiet and uneasy. Nothing dramatic happens, and that is the point: the speaker seems to feel a mild, creeping unease at how healthy something can look while it is failing. The contrast between the pleasant surface and the title is where the poem's tension sits.