It certainly sounds like that impeller wasn't getting water. I would get a new water pump (impeller, cup, wear plate if it's marked up), don't forget to grease the cup and wear plate before you put it back together. The blades on the impeller won't always correct themselves if they're pointed the wrong way so be sure they're correct when you assemble it. Just spin the drive shaft with one hand while lowing the cup. When you test it make sure the water in the bucket is above the water pump. Also don't rev it too much or air from the exhaust will prevent the pump from picking up water.
Pet peeve, it's an anti-ventilation plate. Cavitation happens when water boils from low pressure, usually at the tips of the prop blades. It will erode the prop if it's bad enough. Ventilation happens when the prop gets mixed up with air from above the water or from the exhaust.