One of the founders of the company I work at accidentally left his MacBook outside, and it was attacked by his sprinkler system.
There was some corrosion on the mainboard that had to be cleaned off, and the I/O Board and I/O Ribbon Cable had to be replaced, due to them being fried. But after replacing them, the Mac is good as new!
Use electronic cleaner to get any corrosion off that may have appeared from the water, and be sure to check every inch of each part. Any black marks can indicate an unusable part. And if corrosion keeps showing up, it's best to replace that part.