The world underwater is a place of mystery and mythology, with all kinds of sea creatures, natural and supernatural, that you could include in your project.
Research Tip: See the tips for researching legendary creatures; many of the creatures you will find on those lists are creatures who lives in seas, lakes, and rivers. You will also find some good leads in the Sea Monsters article at Wikipedia, along with lists in the Sea Cryptids and Aquatic Mythology categories.
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