Gulf weed is another name for Sargassum (referred to later in the novel as "Sargasso weed"), a common brown seaweed.
A fathom is equal to six feet or 1.83 meters. Thus 700 fathoms would be 4200 feet (about 8/10 of a mile) or 1281 meters deep.
The term "bait fish" is a generic term for various small fish caught to serve as bait for larger fish.
Squid are cephalopods (similar to octopus) with elongated bodies and ten arms. They are fast swimmers and can grow very large.
When served as seafood, they are called calamari.
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.
So, for example, the English word pen is la pluma in Spanish (feminine). Dog is el perro (masculine), regardless of whether the dog is male or female.
The Spanish word for sea, mar, "is usually masculine, but it becomes feminine in some weather and nautical usages (such as en alta mar, on the high seas)." Thus it is a "noun of ambiguous gender."
Albacore is another name for tuna. "Atlantic tunas vary greatly in size, from the skipjack and blackfin, which rarely exceed three feet (90 cm) in length, to the northern bluefin–the world’s largest living bony fish–which can attain a length of over 10 feet (300 cm) and weigh well over half a ton" (seagrant.gso.uri.edu).
The yellow jack is a somewhat larger marine fish, averaging in length from about 45 cm to 1 m long (18-39.5 inches).
The man-of-war bird is more commonly known as the magnificent frigatebird. The males are equipped with a bright red throat pouch. To see additional photos and to hear the bird, click here.
The Portuguese man-of-war (sometimes "man-o'-war") is a fascinating organism. Its venomous tentacles are extremely painful but not generally deadly to humans.
According to animals.nationalgeographic.com, it "is . . . not a jellyfish, it's not even an 'it,' but a 'they.' The Portuguese man-of-war is a siphonophore, an animal made up of a colony of organisms working together." Its "float [is] 12 in (30 cm) long, 5 in (12.7 cm) wide; tentacles, up to 165 ft (50 m) long."
"Agua mala" is Spanish for "bad water."
Poison ivy and poison oak are two plant species that release urushiol, an oily substance that causes a skin rash when touched.
You can learn more about them from the Poison Ivy, Oak, & Sumac Information Center.
The video below shows the Portuguese man-of-war, the man-of-war fish (mentioned on pages 35-36), and a loggerhead turtle eating a man-of-war.
"Trunk backs" is another name for leatherback sea turtles:
According to cancer.org, "Shark liver oil is promoted as a dietary supplement used to boost the immune system, fight off infections, heal wounds, and to treat cancer and lessen the side effects of conventional cancer treatment."
The towing bitt is a post on a boat to which something towed behind the boat can be secured. The old man uses the term ironically here--he is the bitt (the "post" to which the line is secured), but the boat is not towing something; it is itself being towed by the great fish.
Stepping a mast is the process whereby the mast is inserted into the boat's keel. Hence, an "un-stepped mast" is a mast that is lying in the boat, not vertical. The video below shows time-lapse photography of the mast-stepping process.
Porpoises, like dolphins and whales, are marine mammals--that is, they are warm-blooded and give birth to live young. They are generally smaller than dolphin—"seldom more than six feet (1.8 m) long—and [have] a blunt muzzle instead of a pointed one."