Sean Kingston's female jet ski passenger tells TMZ Sean was driving "REALLY fast" when they crashed into a bridge this weekend ... and insists she begged him to slow down before the accident. TMZ spoke with Cassandra Sanchez -- Sean's longtime friend -- who tells us she arrived to Miami Beach with another female friend to ride jet skis with Sean and his buddy on Saturday evening. Sanchez tells us, "Both jet skis pulled out of the dock at the same time and we were going so fast we just blasted past them. When we turned a corner and crashed ... they were so far behind us they didn't even see us crash."

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