450 stopped mvoing in water

My 450 stopped moving in water that was only 3" over the floor board? I have been in deeper water in the past without any issues. The motor did not stall in any way. It appeared to be the clutch slipping. I thought I needed to replace the belt but the local shop told me to just to pull the plug and drain it. It worked and the Rhino ran fine almost immediatly. Is this just a case of to deep of water or could there be anyother issue that contributed? I noticed the water that drained out was black. It looked like oil and I had to catch some in my hand to confirm it was not oil.