The Tongue of the Manatee is an artifact that allows a person to communicate with manatees while underwater. It is in the shape of a large horn, and the user blows into the base to create the loud noise like a manatee’s voice. It comes with a standard set of phrases that a tourist might commonly use; it is up to the user to learn how to say more phrases.
In his quest for La Esponja Grande, Coronado De Cava knows that he will have to navigate the Manatee Mating Grounds, so he procures a Tongue of the Manatee to get past the giant female manatee who lives in the cave where La Esponja can be found. While he entrusts this task to his favourite crewman Santino, a deep-sea diver who learns the language of the manatees from the Marquis De Singe on Flotsam Island, he considers Santino too important to have such a menial task as holding the Tongue, which he gives instead to Moose. After the party is swallowed by a giant manatee and the crew goes down to its belly to party, Moose uses the Tongue as a funnel to drink copious amounts of ichor.
After Guybrush Threepwood has earned his way into the Democratically United Brotherhood of the Manatee Interior (founded by the four members of De Cava’s crew) and subsequently betrayed their trust, he learns from Noogie (or Bugeye or Moose, depending on which one Guybrush forces the information out of last) the true nature of Moose’s ichor-horn and returns to the Manatee’s belly to retrieve it. With it he is able to talk to the Manatee, but he cannot understand its responses until he finds a way back to Flotsam to get the Marquis’s book on the manatee language. With both these tools, Guybrush makes a deal with the male manatee, pretending to be him and wooing the female guarding La Esponja. He succeeds in drawing her out from her seclusion, and the two manatees are later seen swimming together and holding flippers.