Looks like I need to replace motherboard, yes?

I have a Samsung Galaxy S3 that won't power on. It was my daughter's and got submerged in water. I'm new to this and thought this would be a good exercise, so took out the motherboard in search of the problem, and I think I found it.

Check out the attached images of the motherboard, and especially the first one where I've marked what I think the problem is. Looks like some of the components on the motherboard are fried:

I didn't see any other obvious areas of damage, but I'm guessing that the whole thing will need to be replaced anyway.

So I think my next move is to order the Samsung S3 Verizon motherboard and install it by reversing the disasembly steps that I took to remove the damaged motherboard. Simple as that, or is there more to it?

Also, dumb question: what about the Android OS? Is it stored somewhere on the damaged motherboard that is going away, or elsewhere on the device? If somewhere else, I would think that there is no problem with swapping out the motherboard. But if it is on the motherboard, I have other questions (such as will it be present on the replacement motherboard, and if so, what version, and so on).