It's about a guy (Phlebas) who just dies. That's it. Phlebus does fulfil the profecy, earlier in "The Waste Land" of
Madame Sosostris -- "fear death by water" -- and all that Eliot seems to offer here is the certainty of the finality and decay of death. He even reduces the importance of death by putting Phlebas far in the past (Phoenician).
Anyway, just remember to consider Phlebas...
IV. DEATH BY WATER
By T. S. Eliot
Phlebas the Phoenician, a fortnight dead,
Forgot the cry of gulls, and the deep sea swell
And the profit and loss.
current under sea
Picked his bones in whispers. As he rose and fell
He passed the stages of his age and youth
Entering the whirlpool.
O you who turn the wheel and look to windward,
Consider Phlebas, who was once handsome and tall as you.