Scuba diving
Jacques Cousteau was a Frenchman who developed several important parts of the scuba system and made it useful.  One part was a better regulator that only sent air when the diver breathed in.  This let the divers go farther on one tank.  It was made with a mouthpiece on a rubber hose, so a heavy helmet was not needed. It was light enough to use with fins and easily swim.  He also took many underwater movies and showed people what was under the water and why it needed to be protected.