Hi,
Finally i got a solution, which worked just fine (in my case with Mia Azul). She was not in my household, but this should even work for the case, that you control a mermaid without scale and tail yourself. Unfortunately you have to do that for every mermaid without a tail and scales. You just have to raise their stage of life through the testing cheats (or through natural aging). When they getting older, the game seems to recreate them, sothat the mermaid-state comes into effect. Before or after you do that you can set the stage of life through sim editing, sothat the resulting stage is the stage from where you started. I don't know if the sims need to be onshore, because my sims just celebrated a "birthdayparty". But this only took 30-60 seconds. So if you like experiments: test it in the water. Otherwise just make sure, that your sims are onshore.
Be careful that the sim, which you edit, does not reach the stage "senior" at any time, or he moves like a senior, unless you want a senior mermaid of course. But the senior movement can be fixed, too, through resetsim.
Now she swims like there never was a problem. So I don't need to wait a half day any longer
