Mermaids are not 100 % tied to water, and they can come up on land any time they want to. When this happen, their fins will turn into regular legs with a scaley appearance. They must be regularly hydrated though, so you must shower, drink water, go for a swim and stuff like that pretty regularly.
So, how do you become a mermaid in The Sims 4? The only way to do this is by eating an item called Mermadic Kelp. This item can be eaten by a sim, which gives you a moodlet. Once you have this moodley, go into water to be permanently turned into a mermaid sim. The moodlet will disappear after 24 hours, so make sure you hit the ocean before this happens, or else you will need to get a new Mermadic Kelp.
The next question is obviously, how do you get Mermdaic Kelp? This item has a few different sources, including the following:
- Buy it from the Rewards Store for 500 Satisfaction Points. This will grant you a single Mermadic Kelp that can be used by any sim you control.
- Search in the cave on Mua Pel'am. This cave will have a single Mermadic Kelp hidden in it.
- Get it from dolphins that retrieved it. This is a random item that can be gotten when you retrieve items from dolphins, so it's a nice way to get many of them if you have the patience for it.
- Get it from fishing. This is also random, but you can fish up Mermadic Kelp when you are fishing on any of the expansion pack islands.
And as mentioned in the beginning of this post; mermaids are only available for those who own The Sims 4: Island Living. You will not be able to become or encounter mermaids unless you have this expansion pack.
Create a mermaid in the Create A Sim menuIf you want to create a new family, you have the option to make them mermaids from the very start. Mermaids are considered an occult life state in The Sims 4, so you can choose to opt them to be mermaids when you create the sims.
When you have children, each mermaid give a 50 % chance to pass on the mermaid trait to children. So two mermaids will get a mermaid child all the time, while a single mermaid will give the trait to half of the children (on average).
How to become a regular human sim again in The Sims 4: Island Living
If you regret your choice of becoming a mermaid, you have the possibility of turning your sim back into a regular human again. The only way to do this is by finding two Mermadic Kelp and eating them one after another. This will permanently change you back to a human, and you will need a new Mermadic Kelp to become a mermaid again.