Sometimes the BUD/S trainers will even make the trainees get wet and cover themselves in sand to "make them really uncomfortable," Davids said.
"It sounds laughable," Davids said, but "people's performance cuts off quickly when they're uncomfortable" and it's all about testing whether they can perform under tough situations.
But Davids added that it's not uncommon to witness what he called a "yard sale," in which boats flip, and bodies, oars and helmets go everywhere. Sometimes boats even crash into each other.
"There was a hard and fast rule that during a wipe out each student was to hold onto his paddle," former Navy SEAL Mark Divine wrote.
"Invariably, someone would lose his during a wipe out," Divine wrote. "Some students had teeth knocked out, others had large gashes inflicted on their faces or elsewhere from free paddles."