For a poem about the desert, "The Waste Land" sure has a lot of water flowing through it. And what we're supposed to make of all that water is not always clear. Yes, the waste land is dying from lack of water, but the drowned sailor has also died because of too much water. Water becomes most important in the later stages of the poem, when Eliot focuses more and more on the barrenness of the land, where there "is no water but only rock / Rock and no water and the sandy road" (331-332). It's here that water becomes a symbol of the fertility that the waste land no longer has, and without this fertility, there can be no hope for anything new or beautiful to grow.
Line 4: The "spring rain" comes to bring new life to the landscape; but all it manages to do is "sti[r] / Dull roots," suggesting that nothing new will grow out of the symbolic waste land.
Line 24: This line draws the first connection between the dryness of the land, the lack of water, and the spiritual infertility of the modern world.
Line 47: "the drowned Phoenician Sailor" appears in the tarot cards that the fortune-teller, Madame Sosostris, is dishing your way. He relates to the English myth of the Fisher King, whose wound causes the land to stop producing new life. The drowned sailor in this case might represent the terrible curse that has fallen over Europe as a whole in the 20th century.
Line 55: The warning to "Fear death by water" would suggest at first that you need to avoid dying like the drowned sailor; but fortune-tellers are always full of tricks, and you need to remember that there is a second way to "die by water"—that's if you don't have enough of it. So this warning could also refer to the spiritual drought that has fallen over the waste land.
Line 125: This line comes to us from Shakespeare's The Tempest, and it refers to a guy who drowned and has been underwater for so long, his eyes have turned into pearls. Remember the warning to avoid death by water? Well the turning of eyes to pearls also might symbolize how modern souls have become hard and lifeless. Everything's got a modern parallel in this poem.
Lines 312-321: The entire "Death by Water" section of the poem deals with the figure of Phlebas the Phoenician sailor, whom you were warned about by the Tarot pack. Here water appears to us in the form of a whirlpool (318), sucking Phlebas down into the darkness. At this point, the poem asks us young folks to be a little more humble, since Phlebas was once young and proud, too, and that seems to be what brought him to a watery grave.
Lines 331-359: Eliot gives us what is maybe his most sustained description of the metaphorical waste land of this poem. The most recognizable characteristic of this place is the lack of water. Eliot constantly uses the lack of water in connection with infertility, which conveys to us the sense that the modern world cannot produce anything new or beautiful. Lines like "Here is no water but only rock / Rock and no water" (331-332) drive home this point. When the narrator fantasizes about a better world, he also does so by thinking "If there were rock / And also water / And water / A spring" (347-351), the shortened pattern of the lines almost makes it seem like he's getting delirious with the thought of water, which would bring symbolic health and rejuvenation.
Lines 395-397: The lack of rain has made the river low, and the "limp leaves / wai[t] for rain" the same way that modern people (whether they know it or not) wait for something to give them new spiritual life.