"he heard the trembling sound as flying fish left the water and the hissing that their stiff set wings made as they soared away in the darkness."
Small flying fish
Creative Commons Attribution Share AlikeSmall flying fish - Credit: Dr Jean Fortunet Jdontfight

Flying fish are, well, fish that can fly - short distances, at least. They are found anywhere with warm ocean water. Santiago thinks of them as his "principal friends on the ocean" because he knows when he sees them that the larger fish that prey on them are likely to be nearby.