My wife had a lymphocytic cancer five years ago and she basically flushed her system with water.
90% of her diet was water and vitamins. She also ate cheese for the calcium. When she had an appetite (which was rare), she ate fruits and vegies.
The doctors told her to go home and tend to any loose ends she may have and focus on pain management. She laid on the couch with 2.5 gallons of water and a stirofone cup in anyone sitting. When the water was gone, she would go grab another until she had beaten it.
(She also tells me that there could be adverse affects of having too much water in a normal diet, but, under the circumstances, she said she wasn't worried about that at the time)
-whoisjohngalt