The speaker of "Seasick" sounds like someone who has walked away from the old guides. The captain and the lighthouse, figures of direction and safety, are "done with," and what's left is a startling self-image: "I am a hook." A hook is small, bare, and made for catching, which suggests someone waiting, exposed, and a little dangerous or in danger.
Then the poem shifts to a memory of "a fish, a scholar / at the pint," a funny, slightly absurd figure, half learned and half barfly. This fish named "fen," a marsh, as a home, and did so out of fear of the sea's power. The idea seems to be that people settle for smaller, murkier, safer places because the open water frightens them.
The mood is wry but uneasy. The title fits: the speaker feels unsteady, caught between giving up guidance and respecting how vast and frightening the sea is.