Hello lonut, it appears to be an issue with the LCD or its connection to the logic board. With the tablet being store in a humid location and being splashed, you might want to disassemble the device to look for signs of corrosion or any possible damage to the tablets internal parts such as the logic board and cable ends that connect to the logic board. If you see any signs of corrosion then you may be able to get the device to function again by cleaning the logic board, cable ends and any other parts susceptible to corrosion with a unused toothbrush and 90% or higher isopropyl alcohol. There are guides on this site as well as Youtube that can show you how to clean electronics with isopropyl alcohol. If corrosion is present and you have done a thorough cleaning with the ISO alcohol try and reassembled the device to check if the problem has been fixed. If not the display/LCD may need to be replaced. I would also consider replacing the battery if there were any signs of moisture on the inside of the tablet. As batteries do not handle moisture well and can be the first thing to fail. Batteries can appear to be working correctly after exposure to moisture but in the long run my be a hazard if not replaced.
I hope you can get the tablet up and running again. Best of luck on your repairs.