Associations between water intake and bladder, breast, and colon cancers were found. One study found a significant association between colon cancer risk and water intake in men and women 30-62 years of age. Colon cancer risk was reduced in women that drank 5 or more glasses of water a day. For men, the risk was reduced with 4 or more glasses a day. Another study (2013) found that colorectal cancer risk may be reduced, especially in women, by also consuming no less that 4 cups of water a day.
Cancer patients are also especially susceptible to increased risk of dehydration as a result of their cancer treatment. Several sources suggest that an adequate intake of water for them is at least 40 ounces, while others suggest at least 64 ounces of water a day.
 David, Y., Gesundheit, B., Urkin, J., & Kapelushnik, J. Journal of Clinical Oncology. 2004. Water Intake and Cancer Prevention, 22:2, 383-385. http://ascopubs.org/doi/full/10.1200/JCO.2004.99.245
 Shannon, J., White, E., Shattuck, AL et al. 1996. Relationship of Food Groups and Water Intake to Colon Cancer Risk. Cancer Epidemiol Biomarkers Prev, 5:495-502. http://cebp.aacrjournals.org/content/5/7/495.long
 Tayyem, R., Shehadeh, I., AbuMweis, S. et al. 2013. Physical Inactivity, Water Intake and Constipation as Risk Factors for Colorectal Cancer among adults in Jordan. Asian Pacific Journal of Cancer Prevention, 14:9, 5207-5212. http://journal.waocp.org/?sid=Entrez:PubMed&id=pmid:24175802&key=2013.14.9.5207
 Colon Cancer Alliance. 2017. Colon Cancer and Hydration: Keeping Well, Staying Healthy. https://ccalliance.org/blog/colon-cancer-hydration-keeping-well-staying-healthy/