A "Seahawk" is a nickname for the Osprey, part of the Raptor Family. Other nicknames for the Osprey are "Fish Hawk" or "Fish Eagle"
Actually "Sea Hawk" is not a correct zoological term for any animal, it's only used as a nickname.
The Name was invented by the novelist Rafael Sabatini in 1915. The hero of the novel "The Sea Hawk", Sir Oliver Tressilian, was reverential called "Sakr-el-Bahr" (the hawk of the sea) by his Arabian fellow pirates. The film adaption from 1940 with Errol Flynn was very popular, although not faithful to the book's plot.
Some marine-fighterplanes and -helicopters are named "Sea Hawk" related to this fictional naval hero.