Sub-Mariner #5 (Sep 1968), cover by John Buscema & Frank Giacoia.
Todd Arlis, an Olympic swimmer, suffered serious injuries and accepts taking part in a bizarre experiment conducted by Dr. Dorcas in an attempt to heal. Namor unwillingly takes part in Dorcas's experiment, combining Namor's abilities with the ones of sharks and changing Arlis into Tiger Shark. After the change, Tiger Shark defeats Namor in battle and takes Lady Dorma prisoner.