I've had that exact same thing happen to me. I was living in Ky. My father in law had traded for a gas motor for the boat. I took the boat and motor out to a slough off of the Mississippi near Wickliffe, Ky. It had been a long hot summer and the water was very low. I get the boat in the water and rev the motor up all the way. I'm hauling ass across the water when I see a fish jump. I look over and notice more jumping. Then they started jumping all over the place. Into the boat, around the boat, over the boat. They were flying everywhere, and they weren't little. Some were 10+ pounds. I took a couple home, because I didn't know what they were. Turns out they were grass carp. The only thing I could figure out was that the prop for the motor was just above the mud on the bottom and it was scaring them out of the water. As soon as I'd cut the motor off they would stop jumping. Start the motor back up and they'd start jumping all over again. It was kind of neat, but I was covered in mud and fish slime.