Frigatebirds have the highest wing area relative to body mass of any bird, and correspondingly weak legs and reduced feet. Their plumage is poorly waterproofed compared with that of gannets or petrels. The combination means that a frigatebird settling on the sea risks becoming waterlogged and unable to lift off again.
Everything about their feeding follows from this. They take flying fish and squid from the surface without stopping, pick scraps in flight, and chase boobies, tropicbirds and terns until the pursued bird disgorges its catch, which the frigatebird then takes in mid-air. Tracking has shown individuals staying aloft for days at a time, using thermals and even sleeping in short bouts on the wing.
The piracy is the part that gets exaggerated. Frigatebirds are commonly described as living by theft, but kleptoparasitism is a supplement rather than the main food source at most colonies; surface foraging supplies more. The inflatable red throat pouch of the male is used in courtship display, not in feeding at all.