I’m no expert, but it seems to me that more weight in the raft would increase its speed down the slide, which would increase the likelihood of it going airborne over the second hump. Having people go airborne in a vehicle (the raft) that’s not attached to the slide, and not belted into it, seems problematic from a safety point of view to me. Amusement park / water park rides are supposed to give the sensation of doing something dangerous without any real danger involved.
Sort of like posting on the internet. ;)
The raft clears the second hump, with both momentum and the assistance of water jets. For this reason, a lighter raft is actually more at risk to fly off the ‘hump’.
The heavy rafts just don’t make it up the hump sometimes, and they settle in the low point in the middle. For this reason, there is a short gap in the safety netting, around one raft length, at the low point, for extraction.
People are belted into the raft...with lousy Velcro straps.
There was an unsubstantiated rumor on Reddit, that a woman in the raft damaged her jaw. If true, this would indicate the entire raft lifted up...and that out of the 3 people needed to achieve 400 lb, one was a small kid and another was a woman, meaning the raft was probably right at the minimum.