Literally speaking, it doesn't take a rocket scientist to know that this poem takes place in a boat, on the water, at night, as the speaker heads out to sea. Come one, we've got sandbars, tolling bells, tides, twilight, and all that maritime jazz.
But that's just literally speaking.
If you want to get figurative (and Shmoop always wants to get figurative), you could say that this poem takes place in a kind of spiritual, mental netherworld, somewhere between life and death. It's clear the speaker isn't gone yet because he's making plans for when he finally does set out to sea. But it's also clear that he's no longer quite so attached to the world of the living, because he's totally ready to meet his maker.