Island Living is an awesome pack! Merfolk were designed to have minimal impact on the game, so you can enjoy Sulani without worrying too much about their presence being noticed as much.
I think the game also has only 4 premade merfolk so there’s a good chance you will never see them in their occult form. They can only appear non-human while swimming.
Best way to avoid the occults in this pack is to not go looking for them. Merfolk should appear human as long as your sim hasn’t learned of their existence. Basically, don’t blow the “Ordinary” Conch Shell or “Ask if Mermaid.” Island Elementals will never appear unless you summon them, and the only way to do that is the Child of the Islands Trait. You can remove the lot trait that causes Island Spirits to appear. There’s only one lot in Sulani that has that trait as far as I can remember.
As far as occults are concerned, merfolk have the most minimal presence.
But then again, more often than not, you do need to be looking for occults to run into them.
Some non-occult packs that are extremely enjoyable are:
Get Together
Parenthood
Tiny Living
High School Years
Eco Living
A pack that can let you enjoy the beach without Merfolk is My Wedding Stories. Tartosa is as beautiful as Sulani.