Mermaids are a bit too much of a nuisance for me.
The special diet wouldn't be so bad if when my mermaid got hungry she just ate the kelp in her inventory rather than preparing a regular meal that's only going to half fill her hunger need.
I had a merman child, and once when he was on his way home from school he had dehydrated on the street. I only managed to notice when I selected that sim and saw all of the negative moodlets. I had his father run from home to rehydrate him on the street.
After becoming frustrated with the micromanagement of mermaids I decided to just stay out of the salt water to let the boy and mother become normal. Only a couple hours before final transformation I accepted the free vacation for the parents, and they were gone. The boy became normal, and carries on his life as a land walker now. Upon returning from vacation, the mother had reset the timer on her transformation so I started the process again. Apparently the mermaids are not autonomous enough to rehydrate themselves when they're only a few feet from a shower, because all of a sudden my screen shifted to show me the mother dead from dehydration. Even if the screen shifted to show she had fallen, and needs to be re-hydrated, this would be helpful. Instead you get no notification until it's too late.
For me becoming a mermaid would only be a fast track to level 10 diving before I cure them.
