The best definition that fits the way "moorings" is used in the poem is:
A. definition 1 - the place where a boat is tied up or anchored.
In the context of the poem, "moorings" refers to the physical location where the ship, the Somerset, is secured, which aligns with the imagery of it lying at rest on the bay. The term refers to the ship being anchored or tied up rather than the ropes or the act of tying it up.