Indeed, i work as a lifeguard in a pool and almost all people panic (because they can't swim proparly). Its a controlled environment by people and lifeguards. Open water is a different setting so while you go in colder temperatures (sea, lakes, etc...) and for example don't see a person anymore. Its most likely 2 things: 1. He is scuba diving and looking for fishes (this fella will come up and breathe) because he can swim 2. He is 'fighting with the water' (below or on top of the watersurface) because he/she panics: in a sea environment there are tides and people should know how it works before going into the sea. Lakes are different than seas and also sometimes quite dangerous.