Juliet certainly didn't remember the water being so deep so close to shore before, but maybe they'd travelled further out than she'd thought in their chase.
The light from the surface faded to almost nothing as they dove, the pressure growing higher and the water colder the deeper they went. She trusted Cris, though, which was the only reason she stuck with it and didn't pull out. Even as a mer she was more a creature of the surface, or at least the tropics, made for warm water and bright sandy beaches.
She wondered what could possibly be down this far under the surface.