UNIT 1 WAS A RENTAL SKI FROM MYRTLE BEACH WATER SPORTS. UNIT 1 LEFT THE DOCKS HEADING SOUTH ON THE ICW. THE OPERATOR WAS UNFAMILIAR WITH JET SKI OPERATION AND LET OFF OF THE GAS COMPLETELY AND TURNED THE STEERING LEFT WITH NO THROTTLE, CONTINUED STRAIGHT AND STRUCK A COMMERCIAL VESSEL SEAWRIGHT II IN REAR STERN BOTH PASSENGERS WENT IN THE WATER. Operator suffered internal injuries.