Have you gone to the Marina and clicked on "Think about the fishgirls"? You have to do that to trigger the questline, which allows you to find the first fish girl by whistling in the Nexus.
The second fish girl is in the Mountains. Click on "Explore" and either keep exploring until you find her (it'll happen automatically if you haven't found her yet) or else click on "remember" to get a list of directions to all the places you've been. You're looking for the "lake monster".