Scuba Diving is a type of sport and a method of travel under the water using various equipment to survive under water. This includes a tank to hold air, flippers to move and a mask to see through.
Before going on a dive to the sunken Freighter Lorelei, Cate Archer received some SCUBA diving equipment from Santa at a UNITY training facility in West Germany. There she was able to practice with the equipment.
Archer used the SCUBA diving equipment to travel down to the Lorelei at the bottom of the North Sea in an attempt to recover the captain's log and the shipping manifest. There she encountered Sharks and a team of SCUBA divers from the nearby H.A.R.M. Submarine who were also after the same items.
Scuba diving on Wikipedia