Hi, here are a few things I've noticed about becoming a mermaid, not sure if they're meant to work like this or I am having glitches due to using a few mods.
1. Befriending a townie transformed mermaid is more effective than a real, original mermaid that you have to summon.
You can track them down and invite them to scuba dive together, once underwater, ask for fish parts.
You can ask this again and again and get tons of mermaid kelp, enough to transform the whole household or just keep some for backup.
2. If you are an occult, don't bother to eat the kelp, it won't work and my sim said she felt no differences. Use potent cure first before eating it worked for me.
3. You can use unlock outfits to get rid of those scale leggings in CAS.
However, if you want to use 'change scale colors' interaction to customize tails, wearing scale leggings is crucial or it'd mess up that editing interface, making it unable to change anything.
4. I don't know why, but my mermaid sim somehow stopped boating to a distant area in the ocean, but swim at a faster rate towards it, still, I prefer boats...