Privatizing public water resources means landowners will start dumping trash into Earth's waterways. Pollution will run rampant, much like it did during the Industrial Revolution and clean water will no longer exist. Privatizing water utilities may be helpful because those companies actually make money for high-quality water. Privatizing water reservoirs is not a prudent course of action.
Companies look out for the best interest of the company, not the environment. Profits would eventually, if not instantly, be the motivating factor and less importance would be placed on the environment. Also, the government could have less influence over laws regulating the best interests of the environment if it were privatized.