Minecraft wiki entry for Fishing, associated text: To catch items in the treasure category, the bobber must be in open water, defined as the 5×4×5 vicinity around the bobber resting on the water surface (2 blocks away horizontally, 2 blocks above the water surface, and 2 blocks deep). Each horizontal layer in this area must consist only of air and lily pads or water source blocks, waterlogged blocks without collision (such as signs, kelp, or coral), and bubble columns. These conditions are checked every tick to determine if the player is eligible to receive treasure from a fishing attempt. If the conditions are not met, only items in the fish and junk categories can be obtained from fishing.