I figured it out!
I was having the same problem. I was doing everything right. My household was friends/best friends with at least four mermaids. They hung out for hours on end, at home or scuba diving. I didn't touch the mouse: I let them do their automatic interactions.
Nothing.
Then, as I was navigating MasterController (I have to use Nraas's MasterController, OverWatch, and ErrorTrap to keep the game from being a lag-tastic glitch fest) I realized that the game considered my mermaid friends to be part of my household for some reason. So I temporarily uninstalled my Nraas mods (moved their package files to the desktop), started my game back up, went scuba diving with a couple of my merbuddies, and BLAMMO. Maya Ocean showed me the way to Mermaid Secret Island!
Mastercontroller and the other Nraas mods I mentioned have been game-saving magical blessings from the computer gods, but for whatever reason, they seemed to mess up that one particular interaction. Try temporarily moving their files to another place on your computer and trying again, if you play with those mods and you've been having that problem.