Go to The Explore-Area at Gramps house and go... West, west, south. Follow the creature and do your thing. After that you can go to the marina and "Think about the Fish girls". After that you have to go to the nexus and use the key ("whistle") After that you can interact with them at the marina.
Is there a certain time to do it? I can't actually do anything with fish girl. I see her and she just shows her head and we leave. There's no progression beyond that. I have the Marina unlocked and have basically done everything possible in the game except for one side girl.
