I say both. It can be both, right ?
Poor design because, once again, Disney knows or should have known that a percentage of guests do inadvisable things.
Unrestrained teens on an 8 seater theme park boat attraction that has a mix of thrill and slow movement!!!!......that is a perfect platform for selfishness, stupidity and immaturity to shine. This is highly predictable.
So no it's not Disney's fault for guest stupidity--obviously---but it sure could have designed Splash Mountain boats with restraints knowing guest behavior.