It isn't quite sinking it has a neutral buoyancy between the surface and the bottom.

An excerpt from Cheaper by the Dozen:

“You kids clear out,” said Dad. “But Lillie, if the children can do it, you, a grown woman, should be able to. Come on now. You can't help but float, became the human body, when inflated with air, is lighter than water.”

“You know I always sink.”

'That was last year. Try it now. Be a sport. I won't let anything happen to you.”

“I don't want to.”

“You don't want to show the white feather in front of all the kids.”

“I don't care if I show the whole albatross,” Mother said. “But I don't suppose I’ll have another minute's peace until I try it. So here goes. And remember, I'm counting on you not to let anything happen to me.”

“You'll float. Don't worry.”

Mother took a deep breath, stretched herself out on the surface, and sank like a stone. Dad waited a while, still convinced that under the laws of physics she must ultimately rise. When she didn't, he finally reached down in disgust and fished her up. Mother was gagging, choking up water, and furious.

“See what I mean?” she finally managed. Dad was furious, too. “Are you sure you didn't do that on purpose?” he asked her.

“Mercy, Maud,” Mother sputtered. “Mercy, mercy Maud. Do you think I like it down there in Davey Jones' locker?”

“Davey Jones' locker,” scoffed Dad. “Why you weren't even four feet under water. You weren't even in his attic.”

“Well, it seemed like his locker to me. And I'm never going down them again. You ought to be convinced by now that Archimedes' principle simply doesn't apply, so far as I am concerned.”

Coughing and blowing her nose, Mother started for the beach.

“I still don't understand it,” Dad muttered. “She's right. It completely refutes Archimedes.”