Water water everywhere. And not a drop of it is literal. That's right, folks, in this poem, the ocean is one whopper of a metaphor, representing that Great Gig in the Sky, death.
Lines 3-4: The speaker wants there to be no "moaning of the bar" when he sets out for sea. A sandbar can't really moan, so you might call this personification. It's also a bit of a pathetic fallacy, since this sandbar seems really bummed about something. And what's got that sandbar so down in the dumps? Probably the fact that the speaker is setting out to sea, which in this poem, means he's just about to kick the bucket.
Line 5: When the speaker wishes for "such a tide as moving seems asleep," he's telling us that he's hoping that when he finally does die, it'll be smooth sailing—the tide will be in, and he won't run aground on that pesky sandbar.
Lines 7-8: The speaker describes the tide that "drew from out the boundless deep." The phrase "that which" might also refer to his soul, which is returning home to the sea, or death.
Lines 13-14: By now, it's probably pretty clear that the idea of the flood carrying the speaker far away is a metaphor for death.
Line 16: Here comes that pesky sandbar again. At this point, it's safe to assume that the speaker is being totally figurative here. He's not worried about an actual sandbar. He's talking about crossing the barrier between life and death.