I have a 2002 Mazda Protege LX. I recently went off the road, down into a ditch into water. My front passenger side was submerged for an extended period of time. Tow truck pulled my car out, and it was driven back to my house (25 mins away). My battery light is on, and the battery is charging at 18.5 - 19 volts. It should be charging at 14.5 regularly. Got the alternator tested, and it checked out fine. What could be causing my battery to over charge?

 

I once had an alternator go bad and had it tested. I removed it from the car and took it to O'Reilly's. They tested it on a machine. They actually performed 2 tests at the same time: Alternator and regulator.
If your alternator test was not as thorough as this one then it is still possible that your alternator or voltage regulator are not up to snug.
Another thought: your battery was tilted for the same time as your right front was submerged. Maybe the electrolyte drained from the upper chambers to the lower ones and now your battery's resistance has changed so that the alternator/regulator try to make up for it. If this is still your original battery I would change it no questions asked. If it is newer I would have it tested and I would discuss it with an automotive guy who has grey hair.