More often than not, White Water was likely being evacuated because of a boat jam. That's a situation where 2 boats become jammed between the rotating drums. Not as big of an issue usually as stopping and restarting the drums usually remedies the issue. In the event of an evacuation the Train shuts down because guests have to walk across the tracks if they are being escorted back from the trough right out by Diamond back.
Having spent 2 summers at that ride as the Head Supervisor, I can say with confidence that those boats are way to heavy to ever go air born. And E Stops can happen for a variety of reasons. Someone falling in the water is a given, but there are other mechanical issues that training constitutes an E Stop. The way WWC is designed, an E Stop results in the water level in the station immediately lowering as the pumps would have completely cut off. That being said, when the water level rises, the flow of water generally isn't that strong in the area the boats are designed to collect.