Water Damage: how can I tell what needs to be replaced

Probably u will get more specific answer from somebady alse but I will advise u to take whole Mac apart and look for corosion. If U spot some corosion clean it with isopropanol or alcohol. After that dry it, if it can not turn on teke it to mac specialist.

Hope it helps

Myth number one, drying in rice does anything. Believe it or not, water is not the issue. Well, not water by itself. Water is not conductive without impurities in it. It is those impurities that cause electrons to go where they shouldn't. You end up electroplating the insides of your device with the impurities in the water. As a bonus you can damage the sensitive components too. Now, for why I just educated you. You need to look at the boards with a microscope and see how much junk is on them. They need to be cleaned. The problem is, with how long the device ran with water in it, you could have literally melted SMC parts off the boards. There may now be open contacts. And what is on the board may be shorting to ground.

You could try removing any and all shields, and soaking in wood grain alcohol. This stuff is 100% alcohol, not like the isopropyl stuff you buy at the store (5%). That will likely help. See if it works. If it does, back it up immediately. If it doesn't work, I would drop the board in a Crest ultrasonic cleaner. No, a jewelers cleaner isn't good enough. Then I'd dry it, and check under the microscope to see if anything looks damaged/missing. You may have some micro soldering to do. If you want to know what you are looking for under the microscope there are YouTube videos that will show you. Best of luck, your mileage may vary.