No sooner than King Triton beached Ariel and Arista (for arguing) banning them from going to the Carnival of Tides. Sebastian, of course, is then assigned to be their babysitter.
The Gators arrive. Specifically, Flo and Ebb Gator, freshwater thieves. Their target is the royal treasure, left unguarded in the absence of King Triton's trident. Ariel and Arista quickly develop a truce; the Gators are the biggest threat, not each other. If they work together, they can stop the fiends from reaching the royal treasure. Oh, and Sebastian helps too.
Their characterizations as well as their plan to rob Atlantica palace were most likely a reference to Marv and Harry, the two main antagonists from Home Alone and Home Alone 2: Lost in New York.