The reason they are still swimming with their legs and not their tails is because they have not been outed, or think they have been outed as a mermaid. EA did it this way so for those who want this pack but don't want the occult life state to be "up in your face" type of thing. If you can find one of those large conch shells to blow through. Blow it around the mermaid and they will do a dolphin type of cry, and do something odd with their hands. That is the fastest way to expose them. After doing that, there will be an option to ask them about their odd behavior, or something like that.