Why is my battery not loading / detected?

On the MacBook, the circuits responsible for charging the battery are located pretty much under the middle of the keyboard. Most likely, the water got into these circuits and damaged them. You'll need to have a technician open the system up to examine the motherboard and see if it can be repaired. I actually had this problem happen to me, and I was able to repair a couple of circuits using conductive ink, applied using a single hair (the circuits are nearly microscopic).

Let me note that in my case, it seemed like soda that was spilled, which is much more corrosive than water. Pure water shouldn't actually do any damage, so it's remotely possible in your case that all the water simply hasn't dried up yet. Or it's possible that the motherboard may simply need cleaning. But it's also possible that damage may have occurred.