are there any custom worlds like island paradise with mermaids?

@calvarradosr  As you might well know, Isla Paraiso itself is flawed almost to the point of being unplayable without significant modifications.  A couple of people have cleaned it up and posted their work; here's one thread about it over on the sims forums:

https://forums.thesims.com/en_US/discussion/811253/removing-ip-lag

You can also ask for recommendations over in the forums' world builders section.  There are quite a few skilled builders who regularly post their work, and responders will be able to help you find some well-made worlds with the features you want.

https://forums.thesims.com/en_US/categories/ts3-world-builders

Bluebellflora, who has done more than anyone to make TS3 playable on a mac, has also created several small custom worlds.  She's posted a few on her own site, and you can look up her studio as well.  Her worlds are empty, but you could always download a few mermaids and other interesting sims from the exchange.

https://bluebellflora.com/2013/09/13/archipelago-bay-now-available-for-upload/