@SamsGals99 I think the mermaids need to be at least teenager and up for the tails to show in water; and if they have been dehydrated for too long they are turned to human sims. If none of this applies to your sim: remove any third party CC making sure it is really gone; then delete the caches in Sims 3 Folder, found in Documents>Electronic Arts. They are called:
This is good to do after every play session, and every time you have a problem; they will create anew each time you start the game.
Other things to try:
Bring up the command field by pressing Ctrl+Shift+C at the same time; write: resetsim [your sim's first name and last name], press Enter.
Take your sim to a mirror and Change Appearance: scale colour; then put them in water to see if the tail is showing.
If you have the Supernatural EP: reset their occult state by giving them a Potent Cure Elixir (Alchemy Skill) curing them from their occult life state; and afterwards feed them Mermadic Kelp, turning them into a mermaid again. To delete the cashes in between is a good idea.
Mermadic Kelp you can get from befriending another mermaid and asking about fish parts, under water I think; or by buying the Mermadic Kelp LTR for 25 000 points.