Yes, I had this issue, however it just randomly happened on the first mermaid sim I created in CAS. She wasn't pregnant, and her hydration need bar was full, but the dyhyrdrated uncomfortable moodlet wouldn't go away. It was so annoying that I removed her from that save and played a different human sim in that save for a while. Interestingly though, I started a new save today, pulled that same first mermaid sim from my library, and she isn't having any issues. So I'm sorry that I don't know how to solve that issue for you, but I do empathize. :(