My two cents would say the driveline failed and tried to take the pump with it. I've seen this happen several times in the past. I did exactly the same thing to my pump last year. I just happened to be close to the trailer which was already in the water, so I didn't sink it.
Duane,
What portion of the driveline failed? Mine is out right now, and I will be taking a close look at it.
Brian