By the fact that you dropped your phone in water there is a high risk of component damage and it can be hard to tell which part needs to be changed. My guess is that you need to change the motherboard itself unfortunately since i belive it’s the only electronic part connected to the buttons. If you have a similar phone close buy you can try to change parts between them to search for a faulty part. It is allways worth your time to do a soft reset, factory resets and updates but in this case i doubt this will work in your case. Water damage is allways hard.