Seems like way too much water for either the prop shaft or rudder shaft. Even if you had a near-total failure of the one of those seals it seems like the bilge pump should have been able to keep pace. How long had you been in the water before noticing the water intrusion?

If you were running the boat on the hose just before the trip, Occam's Razor says that's the most likely source of the problem. Do you disconnect the intake hose at the hull fitting, or do you have a hose adapter? One other possibility that you didn't mention is if someone pulled the transom drain plug. I rarely remove it, but I'm always afraid that some a..hole will think it's funny to pull the plug when I'm stopped somewhere. Did you check for that? If all else fails, return to the lake and leave the boat on the trailer while you run the engine and look for the source of the leak. With taking on that much water the source should be very visible.

Let us know what you ultimately find to be the culprit. I frequently leave my boat in a slip overnight and one of my biggest fears is seeing it nose-up the next morning.