Speaking from Wonderland knowledge, which should, but may not be the same - the unload point at the end of the conveyor is right next to the pumps. If the pumps were given a power disconnect, water stops flowing within seconds.
As the highest point on the ride, the water level should drop RAPIDLY. Most of the pump flow is directed away from the load zone (which is why you have to walk over a bridge to get to the station - the water bypasses the station at that point), so it would be relatively safe to enter the water at the unload zone while the water level was dropping, but it should drain in less than a minute.
Based on the first aider testimony, the e-stop wasn't engaged as claimed. Even if the first aider arrived on scene 30 seconds after the radio call, from testimony so far we have in order:
operator presses e-stop
operator calls control to report emergency - assume 1-2 rings, and then 20 second conversation
control sees CCTV, makes the decision to upgrade from 'BLUE' to 'GREY'
Makes the radio call to the responding staff - assume 20 seconds radio message
We're close to a minute after the e-stop has been pressed according to the operator testimony, and allow another minute for the first aider to grab trauma kits, AED and run down the Q4U line - so two minutes to be on scene and the water is chest deep?
Here's an intamin rapids diagram:
And here's a dry station photo of the Wonderland Snowy River (during demolition)
It's clear to see the water level through the station. It is worth noting however that we know Thunder River had issues with stability in the station, and Dreamworld installed the guiderails under the surface. From pics we saw previously, the guiderails were about a foot or so high, so you can probably add another 300 to the above measurements - giving you a possible 1500 deep water at the station - which isn't much higher than chest deep to begin with, depending on how tall you are (average australian male height is about 175-180cm according to the ABS.
My completely uneducated viewpoint - either the e-stop didn't work, or the operator didn't press it as\when claimed.