there are mermaids in sims free play and it seems to be a hobby. You can check out the 2016 January sims free play uodate trailer.
Edit added March 1, 2016:
There was the Tropical Romance Island limited time event that allowed players to unlock a mermaid lagoon. Two sims at a time could swim in there as mermaids/men. They turn back when done, however there is a glitch that allows a way around that. To get a mermaid sim:
- Send one sim in to swim (make sure it is the one you want to be a mermaid)
- Tell another to join them, right before they walk up the steps (to the transformer) they will pause
- At the exact moment the second sim pauses, use the sim tracker to switch to another sim in a different area (if they have started up the steps you will be too late, so have the sim tracker ready)
- Immediately switch back to the sim that you just left (as soon as the loading screen ends)
- If it worked, you will arrive back to find your two sims occupying the same space, one as a mermaid and one as normal (yes, it does look creepy, do not worry)
- Select them and cancel their current action, they will separate. The first sim you sent in to swim should still be a mermaid and you can have them hop around and work on other tasks (or fly brooms...)
- Anything that changes their outfit (Swimming, work, using a dresser etc.) will cause them to change back to normal clothes. Some other tasks apparently do as well (cooking did but woodworking was fine) but so far there is no detected pattern in that (gardening will not change them back).
This glitch works on the Android version of the game, it is unconfirmed if it works on other versions. So far it seems harmless and easily reversed, but do keep in mind that glitches can cause more serious harm and don't run around blaming other people if you mess with it and something goes wrong.