Fish didn't always have tails. They used to just roll around on the sea floor. As they flopped around, they would make a thud thud thud noise. Now there was a sea monster/dinosaur called the taniwha. The taniwha would listen for the thud thud thuds and then come out and eat up the fish. The fish started getting tired of being eaten all the time so they went to Tangaroa (the god of the sea) to complain. They said, "You have to help us, the taniwha comes out and eats us all up!" Tangaroa then said to the fish that they needed to come up with their own solution and he would do it. So the fish got together and thought about it and decided that they wanted legs! So they went back to Tangaroa and said, "We need legs so that the taniwha will stop eating us all!" Tangaroa gave the fish little legs and they went off very happy.
Now the taniwha was listening and when he heard lots of thud thud thuds he came out and ate up all the yummy fish. The fish were very upset that they were still being eaten and went back to Tangaroa and complained. They said, "These legs are no good! The taniwha still eats us up! Take them off!" And the octopus said, "I'll take them!" and the octopus got everyone's extra legs. Then Tangaroa told them "Figure out what you want and I will grant it to you." So the fish all got together and were brainstorming and the seal said, "Oh oh we should have things that are sort of like legs, but just one of them, and all flat at the end and then we can stick it on our backs. That will help us move around better! We can call it a tail!" All the fish liked the idea so they went back to Tangaroa and had him put flippers on the back of their bodies. They were so happy about these flippers that they went and tried them out! Now they were flopping around on the seabed and making so much noise that the taniwha came out and ate them up.
They went back to Tangaroa all upset and asked him to get rid of their tails. The seal said, "Well I'll take everyone's tails!" and put two on the end of his body and two on the sides. Then Tangaroa gave them flippers and when they went to try them out they found that now they could get off the sea floor and swim around in the water. Now that they weren't on the sea floor anymore, they weren't making any noise. The taniwha listened and listened but didn't hear any fish for him to eat. Since the taniwha couldn't hear the fish anymore he eventually got so hungry that he died.
And that is why you don't make loud noises in the ocean or try to yell out to people while underwater. Because the taniwha will come eat you up! :)