The opening paragraph of the novel establishes a central theme: the relationship of England to the rest of the world. Its status as a powerful island empire and ancient seat of learning, a "rock" amid the sea of time and change, is immediately set against the less stable image of England as a "ship"--one which can just as easily be wrecked on the "rocks" of time, change, chance and fate.
As the novel progresses, the metaphors of island and ship are reconfigured, again and again, in relation to shifting contexts. See passages in II.5 and III.4, among other examples. Related keywords for fruitful searching include "wrecked" and "shipwrecked," as well as "bark" and "vessel."