Poison-puffers are small, yellow pufferfish that are found around large islands. There are 4 - 5 poison-puffers per large island so they are very easy to farm once you have mastered the method for killing them.
Poison-puffers pretty much function the same as Creepers do in Minecraft. If you swim too close to them for too long, they will explode in a cloud of poison which will do a lot of damage to you. If you cause them to explode it will not count as a kill so you have to be careful.
In order to kill them you can either use a bow or a spear. With a bow it will take 3 metal arrows to kill them which makes this the easiest method as it provides the least amount of risk to you and it takes the least amount of time. If you want to use a spear it will take 4 stabs from a metal spear to kill them and I recommend using flippers as well. If you don't use flippers while using a spear, it can be difficult to get enough distance between you and the poison-puffer after each stab so that they don't explode. Unlike Minecraft's Creepers, once the poison-puffer starts the explosion animation, it cannot be stopped by swimming further away.