My iPhone 6plus is over heating and screen will not work?

Bean, more than likely the heat is being generated by an electrical short. The minerals in water will cause corrosion that will build up and eventually start to bridge components and connectors that should be isolated. To find out if this is the case, you will have to examine the logic board under a microscope or if you don't have one, then use the strongest magnifying glass that you can find. Their is some protective tape that surrounds the LCD, Digitizer connectors. That tape can be removed from the logic board, do it carefully so that you can reuse it later when reassembling. Once tape is removed you can view the solder joints for the different connectors, Look for corrosion around the LCD and Digitizer connectors as well as all other parts that are on logic board that are not covered with the metal heat shields(both on the logic board and on the end of the flex cable). If you find some, then use a fiberglass scratch pen or soft toothbrush to remove corrosion. Rinse with alcohol.

Water and electricity are a bad combination. If corrosion is not the problem, then you may have board level issues such as a blown filter, ect.