Some TMNT stuff really isn't for little kids.
Captain Yuuki Agamatta was the captain of a Japanese whaling ship, the Shiumi. He and his nephew, Gisei, spotted a herd of sea serpents and caught one of the beasts with a harpoon. When they brought it on board, the mother attacked the Shiumi, killing Gisei and wounding Agamatta.
Using his family fortune, Agamatta rebuilt his missing body parts and rebuilt the Shiumi as a robotic mega ship, the Shiumi's Justice. He took off a revenge mission on Umihaha, with only his first mate, Kai Nezumi by his side.
One day, the Ninja Turtles are cruising around on Shelton Slujj's yacht, when Umihaha attacked it and sank it. The Turtles and the Slujjes take to a life raft, and are eventually rescued by Kai and brought aboard the Shiumi's Justice. Agamatta locks up the Turtles and Slujjes to prevent them from stopping him, but promises to drop them off at the nearest port after his mission.
They are freed by Kai, who says his boss' obsession is consuming him. The Turtles go to stop Agamatta, who sics the Shiumi's Justice's robotic defenses on them. Umihaha appears and shatters the ship, and one of her children helps keep the heroes afloat. Umihaha and the Turtles have a telepathic discussion and she finds they mean no harm. She takes the Turtles, the Slujjes, Kai, and the unconscious Agamatta back to the remains of the Shiumi's Justice, and agrees to tow them to land.