Sounds awesome!!
Some of my ideas:
-If a mermaid kisses a sim, that sim can temporarily breathe underwater. Mermaids can also bring drowned sims back to life by kissing them (only possible if a sim is busy drowning/near drowning/has already died but Grimmy hasn't reaped sim's soul yet)
-Mermaids can sleep in water (swimming pool, ocean, bath etc: they curl up their tail and float on the water, or sleep in the tub with their tail hanging over the side)
-I love the idea mentioned here of bathing instead of showering. It makes sense, as mermaids can't stand in the shower with their tails and Aquamarine is one of my favourite childhood movies!
-Mermaids have perks and weaknesses, like vampires:
Weaknesses:
>Smells like fish
A.k.a. stinks and gives other sims an uncomfortable buff.
>Can only eat raw kelp and raw fish
>Must be in water by sunset
Pops a tail when the sun sets, thus has to sleep in water, or dehydrate and die.
>Terrified of sharks
Will pass out if sim sees a shark and is more likely to be attacked by one.
Powers:
>Control the weather
Mermaids with this power, can alter the weather like @azareasaquarinus suggested.
>Control the ocean
Can make small waves, bubbles of different glowing colours, summon a whirlpool, summon a storm and tranquilize the ocean (makes it completely flat and calm)
>Friend of the ocean
Can talk to all sea animals, fish and sea plants, can ask oysters for pearls, can bring mounted fish back to life, can summon any type of saltwater fish and can order sharks to attack sims.
>Heavenly Sirens
Can lure a sim into the water by singing and then hypnotize them into doing stuff, seduce them or drown them.
>Steal other sims' voice
Mermaids with this power can steal other sims' voices. The voice will be stored in a seashell. The sim will not be able to do socials that use their voice , but can still hug, kiss, play cards etc. silently. The sim's voice will return in 24 sim hours, but the seashell will remain and serves as a sort of button where the mermaid can enchant it again at any time to steal the voice once more for 24 hours.
Etc.