First find the fish girl in the lake, in the mountains, next buy the marina (progress Madison and Veronica storylines to do so) then upgrade the marina, then (if you haven't already) progress the eldritch storyline until you do the nexus keys, the one you are looking for is "whistle" after doing all of these things (in any order) go to the marina and an option will appear to think about the fish girls, after that go into the nexus and put the "whistle" key in, and that should be it. Cheers!