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.