I had a single pedal hobie kayak, now i have a tandem pedal kayak and personally the pedal unit is extremely well built. But, I can see why or how the pedal unit might break, 3 reasons.
1.) They might of not lock the safty pin properly, before going out.
2.) They might of got caught in the kelp and kept on pedaling, forcing it too hard, when they should have unlock the pedal unit and pull the pedal unit out to untangled kelp.
3.) When you come on to the beach after kayaking, once your pedal unit hit the sand, stop pedaling and get off the kayak. If you have a fart brain that day and keep on trying to pedal while your pedal is stuck in the sand you can break the pedal unit.
I have experienced all three situations and if you force it hard enough, I think it will break.
