Tammiejo I have been looking for answers - and was told to ask if you have been testing your world with Testingcheats on because Testingcheats interfers with the Supernaturals so they said it makes sense that would be including mermaids spawning as well.
They did not seem to know if you had been using it on a lot if it would fix itself once you shut it off if you used that left on a lot while building your world. They said since SN is has misbehaved badly with each new ep since and worse than ever and advise to use it sparsely and then turn it off as soon as you are done with it while world building.
I cannot answer any questions as that is just what I was told and I have no clue at all about testingcheats or any other game cheat but MOO as Moo is the only one I have tested extensively, all of them I always turn off the minute I have done with it - so I don't know. But I have looked high and low on this site and two other world building sites and this is the only remote possible answer anyone has came up with because apparently no one has had any trouble getting the merfolks to spawn.
Sorry I couldn't find anything definite - as I really didn't think you would keep that cheat on all the time so didn't really see it as a good answer. And DM says it's not the mermaid special spot - so that makes that useless as an idea. The only other thing I can think of is maybe you are not waiting long enough and are not having sims swimming, diving and snorkeling enough to make them spawn.
The reason I say that is thinking of lots that won't spawn workers sometimes until you send a sim there if they are off in some remote place - well maybe the merfolks need people diving and snorkeling a lot. I generally see the merfolk appearing in all 3 of the worlds I tested that were all unpopulated when my sim had been snorkeling for a couple of days straight after taking diving lessons. Then I see both sharks and merfolks swimming around a bit of a distance from my sims when I look down from over head. They don't come to where my sims is usually though.