What part of the GoPro Hero 4 Black is most likely to be damaged after being water damaged and turned on?

What part of the GoPro Hero 4 Black is most likely to be damaged after being water damaged and turned on?
Added (1). I dropped my GoPro into a river without the waterproof case, after a night of drying i tried to turn it on, it powered on for a few seconds and then shut down involuntarily. Now it shoes no signs of life. Do you think replacing the whole motherboard would fix my goPro or are there other parts that could also be broken and it might not be worth trying to repair it?

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