Associations between water intake and bladder, breast, and colon cancers were found.[1] 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.[2] 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.[3]
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.[4]
[1] 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
[2] 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
[3] 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
[4] Colon Cancer Alliance. 2017. Colon Cancer and Hydration: Keeping Well, Staying Healthy. https://ccalliance.org/blog/colon-cancer-hydration-keeping-well-staying-healthy/