Never See Any Mermaids?

If you have problems seeing mermaids in game and you have any NRAAS mods installed, this may help you:

1) As someone on the Sims 3 forums mentioned, something about Nraas's mods conflict with the NPC mermaids and give them jobs or whatever, so you - almost - never see them.

2) There is a bug in the game with / without the mods, that can cause the NPC mermaids to only appear in human form. i.e. Maya Ocean, Salty Seaworth, even when swimming, you just see their legs.

HOW TO SUMMON MERMAIDS / BEFRIEND MERMAIDS:
1. Download MasterController if you haven't already
2. In game, click your Sim
3. Nraas >> MasterController >> Intermediate >> Long Term Relationship: by Category

4. Scroll down to "Occult Type"

5. Select Mermaid and a list of all the Mermaids in your town will show up. You can select ANY one of them, and select the relationship you want with them:

(Please note the mermaids you see in this list include my own custom made mermaids. It's very likely you'll only 5 - 6 NPC mermaids in your list if you just started playing. The first one should be Azul, Mia)

6. I selected "Good Friend" and then saw Mia appear in the Friendship panel. 
Once you signal mermaid - and you can signal them from anywhere, indoors at home or at the beach... they will appear like a normal guest:

And that's how you befriend a mermaid!
Of course this is definitely cheating and an "unnatural" way to befriend a Sim, but you can guarantee results! You can actually use this method on any Sim, not just Mermaids, so you could play around with the controls... you know, like when Love Potions fail - or something. ;)

I only tried this method after hours of unsuccessful play time, and I couldn't find a way to reassign merchants that were mermaids (as someone else claimed on the forums) because I didn't even know WHO was a mermaid to begin with! I hope this guide helped you, even though it's very late, there just might be other players late to the expansions like me.