From what I can tell, the problem wasn't, as such, that the poor girl fell out of the boat. She seems to have managed to get up just fine, and even spoken to bystanders. She must have been soaked, but okay at that point. But a guest-out-of-the-boat situation should be treated as a dire emergency, and it wasn't. It seems she was left to find her own way out of the attraction, and ended up in deep water where she tragically drowned. And it still took ten minutes before anything was done about it. That's the major bleep-up committed here. If there even existed any routines for what to do when somebody falls out of the boat, they clearly weren't followed. When such a thing happens, it's all about stopping everything and getting a staff member to assist the rider immediately, because a rapids ride can be a very, very dangerous place to be. Rushing water deep enough to drown in, freely floating rafts weighing hundreds of kilos, and partially submerged, moving machinery. It's one pyrotechnics effect away from checking off every single hazard on the OSHA bingo board.
In a risk management model, a guest falling out of a boat would be considered a "hazardous event". It's generally something you want not to happen, and you have barriers in place to prevent it from happening (proactive barriers), but it should not automatically lead to disaster. There should be reactive barriers as well, and these appear to have been inadequate or failed on that fateful day.
It's like someone falling into the tiger enclosure in a zoo. The fall is not necessarily the dangerous bit, it's what happens afterwards. Given the nature of a rapids ride and the somewhat unpredictable behaviour of the average park-goer, you can't really assume that no falls will happen. It becomes even more imperative to have a plan to deal with that situation, and Drayton's was clearly not good enough. That's what they're being sued for. Not for someone falling out of a boat, but for an otherwise survivable fall leading to a death.