Watermelon Tourmaline is a giant, pufferfish-like monster and the main antagonist of the Steven Universe episode, "Beach Party".
In "Beach Party", Watermelon Tourmaline attacks the Crystal Gems, deflecting all of their attacks and literally blowing Garnet further into town. Amethyst then transformed into a baseball bat and knock it across the ocean.
It later turns up during the Gem vs. Pizza Family Volleyball game, endangering all of them. Nanaphwa and Steven then work together, organizing the others into helping and they ultimately defeat the creature by popping it on a giant shard of glass.
Watermelon Tourmaline is now currently inactive and bubbled within the Temple basement.