There are many things that can happen when water (or any liquid) enter in contact with the logic board and you lose the backlight function.
1- the LVDS socket on the logic board could be shorted and damaged cutting the electric flow to the screen backlight. NB: the video cable connector could also may be damaged by the socket short. Visual inspection must be performed to check it out.
2- the backlight fuse located near the LVDS socket may be blown by liquid spill. Check fuse continuity with a multimeter.
3- transistors, resistors near the LVDS socket or the charger IC may be shorted. You need the board schematic and the board view file to troubleshoot the components.
4- Charger IC could also be the culprit. The IC is 28 pins package (most of the time named U7000). Not easy to replace.