Mermish

Mermish is the native language of most Merfolk . Sea Common is built out of several languages, but it's spoken form is more heavily based on Mermish than any other.
To outside observers, Mermish seems to have a vaguely lyrical quality.
The Mermish basic written language is pretty simple. There are a few symbols used for signs or map markings and a lot of symbols for written music.
If the Mermish want to write something complicated and nuanced in Mermish, they usually use an adaptation of the Asta'lash script.