You became a mermaid/merman after a failed attempt to crash from a cliff jump, instead you drowned so the sea granted you a tail. After spending some time in the sea you learnt to survive there as there was no other way out. Either learn or become the dinner of some shark.

One day you just ran into a pack of sharks, and at high speed swimming away from them you got into the nets of the ship. When you were lifted up on deck and thrown out of the net with the fish. You heard the clacking of heels on the wooden deck and the sultry laughter of Captain Kraken. "What a precious catch we have today..." Robert said exhaling tobacco smoke from his pipe. He walked over and picked you up in his arms, saying to his crew, "Why are you standing there like statues?! Prepare a barrel of water in my cabin."