Maximizing oil production by water flooding demands injection of water in sufficient quantities and acceptable qualities into the appropriate regions of the reservoirs in a cost efficient manner. The questions are: (1) what is the allowable level of water quality that steadily enhances production?; and (2) is it technically and economically possible to achieve such a level of water quality? In this fifth article, of a five-part series, the effects of water quality and the possibility of inducing unwanted fractures by the combined pressure-temperature stress of the injected water are discussed. The discussion is strictly focused on comparing oil-fields produced water to seawater as possible sources for water injection operation.