I was flying my Phantom over the ocean this morning. Everything was going fine. I flew for 8 minutes on the first battery and had no problems. I even used the GPS home feature and it was fine. I was 8 minutes into flying with the 2nd battery and it went down in the water. It even flipped on it's back right before it went into the water. I had the RC on GPS mode. I tried to increase it's altitude as it was going down but nothing was working. At the last moment I tried to use the GPS home feature but it was too late. It was 1/8 of a mile from me but I have flown it 1/2 mile before with no problems. I don't understand why it went down.