You posted your question in March, so you've probably finished your project by now -- but for what it's worth: arguably the most famous reference in Poe to "water" is from Annabel Lee:
Code:
For the moon never beams without bringing me dreams
Of the beautiful Annabel Lee;
And the stars never rise but I see the bright eyes
Of the beautiful Annabel Lee;
And so, all the night-tide, I lie down by the side
Of my darling, my darling, my life and my bride,
In her sepulchre there by the sea--
In her tomb by the side of the sea.
The sea in the poem works in opposition to the sky and heaven. Other references to "water" or the sea rather include "Night's Plutonian shore" in "The Raven" or "the dim lake of Auber" in "Ulalume".
I am not sure to what extent water is a significant symbol in Poe. Lovecraft would be more interesting in that respect 
Regards,
Raven.