Water damage - how to diagnose specific problem?

You're going to want to start with an ultrasonic cleaner, or at least a deep clean with isopropyl alcohol (91% if possible). Don't use a jewelry cleaner but instead a true ultrasonic if you have one.

Strongly recommend you desolder the emi shields on the motherboard itself to give it a quality cleaning.

Basically what you're looking for , first at least, is to remove any and all corrosion buildup on the board. When water or liquid sits it leaves minerals and deposits on the board. These deposits (corrosion) can add extra pathways for current to flow through including directly to ground...which is not where you want most components to go.

I would first do a cleaning then let it get an air dry for about 30 min to 1 hour and inspect the board. Look for physical damage such as damaged pins on connectors. If you get a schematic online you can also test main lines for shorts using continuity mode on a digital multimeter.

Also, unless you're doing this as a pure hobby I strongly advise against putting a lot of time into it for any reason other than data recovery.