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Ronda Rousey has already mastered the "Battle of the Network Stars" obstacle course -- but her friends ... well, they BOTH ended up in the water pit during a test run on the L.A. set.

TMZ Sports has footage of the UFC legend coaching her mom and sister as they tried to navigate everything from the rope swing to the monkey bars.

Ronda easily conquered the obstacles at Pepperdine University -- her fam had a much tougher go. Fortunately, Ronda was there to pull 'em BOTH out of the water.

Ronda is a permanent "team captain" on the show -- and every week, she'll lead a squad against DeMarcus Ware's team.