temp warning on batteries can be caused by a few different things. moreso since it has water damage. in order of operations, try these repairs
1) Corrosion cleaning: White film will build up on the device with water. this is conductive, and may create short circuits, causing it to draw too much current, and overheat. it spreads over time.
2) Battery fail: The battery may have issues with its integrity. replacing the battery will fix the issue.
3) Charge port fail: The charge port flex houses the Circuit that processes battery temperature. if that circuit is bad, then you will get the error specified.
4) Motherboard failure: if the motherboard has failures processing the signal, then likely you will have to replace the motherboard. this process is usually more expensive than it is worth, and i suggest replacing the device at that point.