Serusan

Serusan (or Serosian common) is a shorthand language used as a common trade language by shallow-water races. It is a distinctly an undersea tongue, it contains dolphins' clicks and whistles, words from the merfolk language, the aquatic elvish tongue, the shalarin tongue, some parts from locathah, kuo-toan, and sahuagin speech. All the races noted above can speak and understand Serusan, but unless fluent in it as a separate language, they can only grasp the most basic meanings.

As per Sea of Fallen Stars (TSR 11393)