Friend Tried To Prevent Sean Kingston Jet Ski 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."

Sanchez says ... as Sean was racing towards the bridge, she screamed out, "We can't fit under there?? Are we going to try and go under there? Sean stop!" Sanchez tells us .... she believes Sean tried to turn away from the bridge at the last minute ... and lost control because they were going "REALLY fast."