The first part of book 3 of Ysabel the Summoner takes place underwater. This series is a YA fantasy, so the more fantastical the creatures, the better.
Enter the hammerhead, prowler of the deep.
The facts: Great hammerheads can grow up to 20 ft. and weigh up to 1k lbs. Their big eyes capture the dimmest light, allowing them to see in the dark. Sweeping their popeyes back and forth gives them a panoramic view. They detect electrical impulses given off by prey hiding in the dark and under the sand.
What was that?
I hope never to see this coming at me.
Or looking up at this.
Next are Japanese spider crabs, aka tall-footed crab. My girl is going to meet some scary creatures.