What to do if my MacBook is not turning on due to water damage?

1 - Keep the computer Off and disconnected from the Charger

2 - This may help avoid am Electrical Shorting Out causing with More Damages and could avoid a possible Fire

2 - Have the computer evaluated by the Professionals who have the Special Hardware Software and Expertise to diagnose the issue and offer possible solutions

3 - Make an Appointment at the Apple Genius Bar and have them evaluate the computer

Only them can determine what costs maybe involved or if the machine can be repaired

I cannot tell you if it is completely gone without having a look on the hardware, So I should take it to Apple if I where you and they can tell you if it is repairable or not