It is not clear what you mean by "impact." With hundreds of desalinization plants worldwide (California has even built a couple of them), seemingly the impact is well understood:

Salt water is converted into fresh water and trace elements and minerals. Said minerals may be returned to the fresh water to prevent mineral loss in humans drinking said water. Some filters require changing to prevent organisms from entering the intake lines. An accumulation of salt from the process may be packaged and sold or returned to the ocean.