flip·per 
/ˈflɪpɚ/ noun
plural flip·pers /ˈflɪpɚ/ noun [count] 1 : one of two flat body parts that stick out from the side of a seal, whale, etc., and are used by the animal for swimming 2 : a flat rubber shoe that has a very wide front and that is used for swimming — see picture at scuba diving
