In almost all contexts, water in Purgatory signifies purification. The one exception is the sea surrounding the island. If it were possible to be purified in that, who in his right mind would climb the mountain? The idea of water as a cleansing agent is rather intuitive, especially given that Purgatory is a place for cleansing or purgation. In case you’re not convinced, let’s take a look at a specific instance. In Canto I, Virgil bathes Dante’s face with dew at Cato’s command before girding Dante with a new rush belt. The lines go:
When we had reached the point where dew contends
with the sun and, under sea winds in the shade,
wins out because it won’t evaporate,
my master gently placed both his hands –
outspread – upon the grass; therefore, aware
of what his gesture and intention were,
I reached and offered him my tear-stained cheeks;
and on my cheeks, he totally revealed
the color that Inferno had concealed. (Purg. I, 121-129)
Virgil uses the dew from the grass to wipe away the tears from Dante’s cheeks and “reveal the color that Inferno had concealed.” In other words, he cleans all the dusty filth from Hell off Dante’s face to reveal his true complexion beneath. If on the purely physical level, the dew washes away dust and grime, we might also draw an inference that it’s washing away the emotional remnants of Hell as well – the fear and desperation of the damned.
Now for the rivers: Lethe is the Classical river of forgetfulness. That’s right folks, Dante didn’t invent it, the ancient Greeks did. But Dante has a way of giving all things Classical a Christian spin. When all the vices have been purged from a soul by the rigors of Mount Purgatory, the soul undergoes the last round of purgation by being immersed in the river Lethe, where even the memories of sin are washed away. Talk about the ultimate purgation. Once a person has gone through the Lethe, he can’t even remember his sins on earth. It seems as though Heaven doesn’t want even the slightest thought of evil in its celestial realm, so the Lethe takes care of that. Once that’s done, the soul is immersed one last time in the river Eunoe, which restores the individual’s good, virtuous memories.
Interestingly, there is one more blatant symbol of purgation, which has nothing to do with water. Actually, it’s the exact opposite – fire. Remember that on the seventh and last terrace, Dante has to walk through a wall of fire to purge himself of the final sin of lust? Instead of soaping up and gently rinsing away, lust must be burnt clean off.