Fin and flipper locomotion

Fin and flipper locomotion occurs mostly in aquatic locomotion, (and rarely in terrestrial locomotion) of nektonic animals with paired appendages, more specifically aquatic vertebrates (fish and marine tetrapods) and most coleoids. These appendages have a large wetted area that interacts with water like paddles and/or hydrofoils to propel, steer and maintain postural control/balance of the body during locomotion.

Source: Wikipedia — Fin and flipper locomotion (CC BY-SA 4.0)