Death by Water Summary

Get out the microscope, because we’re going through this poem line-by-line.

Phlebas the Phoenician, a fortnight dead,
Forgot the cry of gulls, and the deep sea swell
And the profit and loss.

Welcome to the shortest section of the poem, called "Death by Water."
These lines tell us that some guy named "Phlebas the Phoenician" is the one who's been killed by water. He's been dead for two weeks, or a "fortnight" (though if he really is a Phoenician, he's been dead a lot longer than that).
Phlebas is probably connected to the "drowned sailor" from Madame Sosostris's tarot pack, and for Eliot, the image of him drowning is…well…unclear.
You could say that Phlebas' death is necessary before spiritual rebirth can happen; you could also say that death is death, and that's it.
When these lines talk about how the dead Phlebas "Forgot the cry of gulls […] And the profit and loss" (313-314), they suggest that Phlebas, now dead, doesn't really worry about worldly things like making money anymore. Eliot will expand on this idea in the coming lines, so stay tuned, Shmoopers.
Formally, "Death by Water" is definitely the most organized and structured of the five sections of "The Waste Land." It's spaced as ten lines, but when you read it out loud, you can hear quite a few rhymed pairs in it ("swell/fell," "Jew/you"). The language of the section is also pretty formal and old-timey, since this section is basically like a classic parable or story intended to teach us an important lesson about pride. Don't worry—we'll get there.

A 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.

Just imagine that hollow, droning sound of the ocean as your dead bones get picked clean by "whispers" of seawater for years and years. So creepy, right?
The next lines say that Phlebas "passed the stages of his age and youth / Entering the whirlpool" (317-318). This might refer to the idea of "your whole life passing before your eyes" that usually gets associated with the moment right before you die.
This could further refer to the human brain and how it tries to make sense of your life only after it's too late to do anything about it.
For Eliot, the same might be true of modern people; it's only after they're on the brink of death that they finally take stock of their lives and think deeply (about just how shallow they really are).
The image of the whirlpool could be the drain that modern culture is slowly circling around, ready to sink down into darkness forever.

Gentile or Jew
O you who turn the wheel and look to windward,
Consider Phlebas, who was once handsome and tall as you.

The speaker makes a call to people of any religion, whether "Gentile or Jew," and says to anyone who sails confidently over the sea of life (or "look[s] to windward"), "consider Phlebas, who was once handsome and tall as you" (321).
In this case, Phlebas becomes a cautionary figure for anyone who walks around thinking they're awesome, since there are many people just like them who've died in the prime of their lives. It wouldn't hurt to be a little more mindful of the fact that one way or another, you're going to die someday.