Car Will Not Start problems of the 2007 BMW 530

Last year I could hear a sound of water through my sunroof and would have water thrown at me through my ac. My stereo and dash stopped working and suddenly started working after a couple of months. I took it to 2 different locations and they would tell me there was no short in a fuse. Yesterday my car would not start it was stationed in front of my home. I went to a mechanic and found that my battery was sitting in water. The car will not start even after water was drained and the battery was replaced. I called BMW but they want to charge me $385 just for a diagnostic. I have changed my battery like 4 time in the last 2 years which is just a headache in itself. My vehicle wont start.