Minecraft wiki entry for Achievement_guide, associated text: Hold an empty bucket or water bucket, then press use on a cod, salmon, pufferfish, or tropical fish. Cod can only spawn in lukewarm, normal, and cold ocean (along with their deep variants). Pufferfish and tropical fish spawn only in warm oceans. Salmon spawn in cold, frozen ocean (along with their deep variants), river and frozen river. So, salmon are the easiest to encounter, since rivers are easier to find than oceans.