The image of the "cold gray stones" could suggest the cold gray <em>tomb</em>stones of a cemetery.
And the stately ships go on To 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 <em>The Addams Family</em>…
The sound of a voice that is still. (12)
Again, the speaker longs to speak with his dead friend, but he doesn't imagine chatting with him – he only imagines a disembodied "voice" that is now "still." Instead of imagining his friend in his entirety, he imagines him only as a series of absences.
A day that is dead (15)
This is the only time the word "dead" comes up in the poem, so it must be important. But the speaker doesn't describe his friend as "dead" (only as "vanish'd" and "still") – it's the time that he spent with the friend that is "dead" and gone.