Journal Scrap #9 mentions a mermaid sighting by a beach in the rain. The beach the note refers to is the beach near the pirate cove. You'll need to rebuild the beach resort to access the southeast portion of Ginger Island, which is where the mermaid can be found. On this part of the beach, you'll find five areas with rocks where you need to place flute blocks. A flute block can be made after getting the recipe from Robin through her 6 Heart event. Once you have five, place one directly under each of the rock clusters on the beach. Next, you'll have to tune the flute blocks according to the rocks in the rock cluster above each one. You can tune a flute block by clicking on it. A small rock equals 1 click, while a big rock equals 5. Tune the block, then run across them, starting from the left. This plays the Mermaid's Song, which results in a reward of 5 Golden Walnuts.