You can call them "scuba diving fins" or "scuba diving flippers". Scuba divers use these on their feet to help swimming at the surface and below the water. These days divers have multiple styles to select from.
These days high tech swim fins are a unquestionable marvel of engineering and the physics of pushing underwater. Using modern mathematics and C.A.D. (Computer Aided Design) techniques, most types of scuba diving flippers display hardly any resemblance to the regular paddle or full blade scuba diving flippers. These strongly angled or split style fins were designed to maximize effectiveness and slip the scuba diver under the water with minimal effort, increasing dive time. The best estimate of how good a scuba diving flipper works is not its design or effectiveness, but its comfort -the best designed scuba fins wont pay you to get them if you are not comfortable using them.
Whether the every day paddle or full blade style, or the more up-to-the-minute split and cut out designs, no matter what the design, scuba diving flippers are basically divided into these 2 categories, "Full Foot" or "Open Heel. The Full Foot flippers slide onto the foot like a shoe or boot, "open heel" fit with an adjustable strap around the heel following the foot is slipped inside. Many scuba divers favor the open heeled types for the versatility. They may be worn with other accessories like booties, able to be used in a a larger range of diving levels. If you know you are only going to be diving in warm waters, and will not need to wear booties, then the full foot flippers may be a more appropriate choice, they can be a little tighter fitting, and lighter then open heeled flippers, and are much easier to maneuver. Full Foot scuba diving flippers are used more often by snorkelers.
Multitudes of scuba divers still prefer the regular full blade or paddle scuba diving flippers, yet even these have evolved since their start, being built from better and more yielding materials. The answer to how a fin performs is the size of its blade the more extensive the blade the added thrust but the more force you will run through to move it. A large number of new styles of fins, try to maximize thrust and minimize energy expended by cutting vents through the blades, switching angles, and/or splitting the blade. The split blade is one of the most popular of the new fresh designs. The split scuba diving flippers also called bio-fins, dramatically enhance the dynamic of a single kick, and can curtail the magnitude of kicks needed to encompass equal distance using a traditional fin. Distinct new designs in scuba diving flippers fashion vents into the blades in addition to the splits, that push water out with each additional kick, thus these scuba fins move more like a propeller than a paddle.
Which ever type of fins you decide on you should always verify them for fit and comfort like in a pool first, before taking them out to the deep water. A flawless fitting pair of expeditious scuba diving flippers is a piece of gear all divers should own, and will help you get a blast out of scuba diving.