Minecraft wiki entry for Guardian, associated text: 40% chance of dropping a raw cod (cooked cod if killed by fire‌[JE only]). An additional 0-1 drops are added per level of Looting for a maximum of 4 cod with Looting III.
40% chance of dropping prismarine crystals. An additional 0-1 drops are added per level of Looting for a maximum of 4 prismarine crystals with Looting III.
20% chance of dropping neither.