Fish for clues is a story quest within Moonglow Bay that is part of The Monster of Moonglow main questline. After investing 300 seashells into the Cozy Cavern, the protagonist speaks with Elvice inside. He tells you about some destroyed ruins near the Perching Isles, which might hold clues surrounding the town’s elusive monster.
Upon visiting the location, Moonglow Bay highlights the ruins and removes the quest marker with the description telling you that you should fish for clues because they might have left something behind. Naturally, some players might end up wasting time fishing the Perching Isles waters. Others might think to use the net to pull the ruins in as the game has already taught players to use it on land.
The actual solution is much simpler. Park the boat in front of the land mass with the ruins, then aim your fishing rod at the center of the sparkling ruins. Reel them in with the right trigger and pull back with the left stick. Unlike with the fish, there won’t be a visual indication of how far along you are. The ruins will just magically appear on your ship after a second or two of pulling back on the reel.