-As implied, when mermaids swim, their legs turn into fish-like tails, letting them swim faster.
-They cannot drown.
-If Seasons is installed, they will not need umbrellas, as the Soaked moodlet is positive for them.
-They can scuba dive indefinitely, and will not get the depressurising moodlet.
-Sharks will not attack them. Ironically, if they have the mean spirited or evil trait, they can summon sharks on command.
-Their hygiene need is replaced by hydration.

-Like werewolf Sims from the Supernatural expansion pack, they can eat raw fish.
-Mermaids can eat kelp, which can be harvested from diving areas or purchased from the grocery store. Kelp can also be a choice for their favorite food.
-When Mermaids are on land, it is still possible to tell them apart from normal Sims just by looking at them. Their visual difference on land is that their legs have scales instead of skin.
-All mermaids have a maxed out Scuba Diving skill.
-According to onlyabidoang's Mermaid Abilities video on YouTube, Mermaids swim at the fastest speed out of all life states.
-They do not need umbrellas.
-Hence the first advantage mentioned, fishing is a useful skill for mermaids when it comes to filling up the hunger need.
-Their hydration need is easier to fill up than hygiene, as mermaids need to stay wet.
-Oddly, they can still have the Loves the Heat trait, despite not wanting to go out in the heat.
-Zombies (Supernatural) will never attempt to attack them.

-Quite tough for a Sim to become a mermaid, as they must be good friends with an NPC mermaid and ask them for kelp whilst underwater or get the kelp from a 25,000 point lifetime reward.
-The said item above cannot be purchased using the buyDebug cheat in the console.
-Bot fans (Into the Future) cannot become a mermaid.
-Unlike the Island Living expansion pack from The Sims 4, they cannot be saved to the bin or start as mermaids in Create-A-Sim, as the supernatural type menu (Which requires the Supernatural expansion pack) does not include standard occult life states from later expansion packs.
-They cannot get a tan if they have the pale mermaid skin tone. (Seasons)
-They cannot have the Hydrophobic trait, unlike PlantSims (University Life), who very oddly can still have it.
-Cats (Pets) are more likely to follow mermaids, which can be annoying.
-They will lose their mermaid abilities and become a normal Sim if they do not swim in salt water for too long. This can be countered if they have the Permanent Mermaid lifetime reward.
-A low hydration need can lead to unconsciousness or death if not filled.