Was the computer on at the time, or has an attempt been made to turn it on? The first thing to do is open the case, dry everything out, put it near a small fan if you can, don't use heat. Don't try to turn it on, that would be the worst mistake possible, until it's completely dried out. Let it dry for a couple of days if possible.
Water by itself will likely only damage the hard drive, most of the stuff in the machine was washed with pure water, in some cases many times, during the manufacturing process, as long as it isn't turned on water will not damage it.
You can also take out the hard drive, if it's dry you can use an adapter to hook it to a desktop machine as a slave and see if the files are intact.