The speaker describes a life that feels cramped and unpleasant. The word "rank" suggests something stale or gone off, and "thin" makes it sound underfed, lacking substance. There's no one else in the picture: the speaker has "nothing to care for but myself," which sounds less like freedom than like emptiness.
The third line is the strangest and most striking: "sea surge in a drowned mouth." A drowned mouth can't speak or breathe, yet the sea still rushes through it. It's easy to read this as a sense of powerful feeling or energy with no way of getting out, or as something forceful that has become meaningless because there's nobody left to hear it.
The last line, "capable of being what I do," is open-ended. It might mean the speaker is defined only by their actions, or that they could be more but settle for less. Either way, the mood is bleak, quiet, and self-aware, with a faint note of restless potential underneath.