On thy cold gray stones, O sea! (2)
The image of the "cold gray stones" could suggest the cold gray tombstones of a cemetery.
And the stately ships go onTo their haven under the hill (9-10)
The "haven under the hill" sounds rather macabre – it suggests burial mounds in cemeteries, where the dead find rest or "haven." If so, then the wooden "ships" might represent wooden coffins moving steadily toward burial. This kind of makes us think of the elves in The Lord of the Rings heading off to the Grey Havens (though Tolkien wrote than many years after Tennyson wrote this poem).
O for the touch of a vanish'd hand (11)
The speaker longs for the company of his dead friend, but he doesn't imagine the whole guy – he only imagines this disembodied, "vanish'd hand." Kind of like "Thing" in The Addams Family…