Technically, any time a Guest exits the RV while the ride is in operation, an E-Stop should be depressed. If someone looks like they are about to exit, or thinking about exiting, a Station Stop should be depressed.
It's a pain when an E-Stop is hit becuase it takes anywhere between 20 and 45 minutes to get the ride running again. The water drains from all three circuits and the techs have to re-enable the ride by running to every control panel on the ride in a certain sequence.