Soaking the motherboard and other electronic components in fresh water is far less threatening to the consequences than in salt water. I currently have a Mavic Air drone in my workshop after being submerged in the ocean. From the outside, the drone seemingly looks undamaged, however, after opening the body, it turns out that the effect of salt water on the motherboard is very clear.
It manifests itself as follows:
- all sheet metal shielding elements are heavily corroded, often even disintegrating,
- some paths on the PCB are so corroded that circuit breaks appear,
- most sockets and plugs are so corroded that they no longer conduct electricity,
- the most severely damaged is the battery - if salt water gets inside it will damage the connections between the battery cells.
Returning to your question: the camera, motors, GPS module, IMU module (the latter, if present, is usually tightly encased in a rubber cover) may have survived. On the other hand, in all likelihood, the main board, the ESC motor board, the TX receiver, the video transmitter could have been damaged.
Identification of damaged components is primarily a visual assessment for signs of corrosion (time to dry out moisture is unfortunately often enough to corrode components intensively). In the drones that I had the opportunity to service, the entire surface of the motherboard after flooding with salt water was covered with a thick layer of white corrosion. This tarnish should be washed off with isopropyl alcohol, and then under a magnifying glass examine all components (especially sockets and plugs) for signs of corrosion.
If, after all, the electronic components have not suffered visibly (no visible signs of corrosion), you can risk connecting the power supply and assess the drone's behavior after switching on.
P.S Apologize for any language errors - I live in Poland and use one of the translators.