With water damage, you have to decontaminate the logic board before doing anything else, otherwise there will be latent issues down the road.
Not every water-damaged phone is recoverable and depending on the kind of liquid (salty, dirty, filthy), sometimes the success rate is quite low. That said, IMHO, it's always worth a try otherwise we are just piling up the e-waste.
Here's what you need to do:
- Open your phone and remove the logic board (follow this guide)
- Inspect the logic board, especially around the connectors and look for corrosion.
- Inspect both sides of the board. Unfortunately, 80% of the board is covered in shields. That's usually where the damage is occurring.
- Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
- Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
- Rinse in alcohol and repeat.
- Let it air dry for a day.
- Re-assemble and hope for the best.
You should also replace the battery if it has swollen. Resist the temptation to pop it to let the gas out. A compromised Li-ion battery is a fire hazard. If the device appears to power up but behaves erratically, then use a tool like 3uTools to flash the firmware as it may be corrupted.
A professional repair shop that does water damage repair may be able to recover your phone or the data because they have access to pro-level ultrasonic baths and specialized cleaners as well as the skills to troubleshoot your board. Many shops have a no fix/no fee policy so you don't have to spend money to find out if the phone is fixable or not.