Minecraft wiki entry for Advancement_guide, associated text: You can also find one in ruins chests, obtain one from a drowned, or buy one from a Journeyman-level Fisherman. Now find a body of water and throw the bobber in the water. There will be bubbles moving next to the bobber, indicating that something is in the water. Once the bobber dips in the water, quickly reel it in. To get the advancement, you need to catch a fish, not treasure/junk items (you can only catch treasure loot by fishing in open waters). If you didn't get a fish; try again.