Years travelling with the Doctor, running with the Doctor and keeping up with her older at the time partner had left her in pique physical condition. She liked it that way and she knew her, currently younger, partner liked it too. It wasn't that she had a sixth sense about these things but more an intimate knowledge but she wouldn't let Sam know that. Just yet. Spoilers as River would say.

"There, isn't that better?" Coraline asked Sam, pretending not to look until he was in the water. Swimming over to him, she smiled as she turned in the water. Accidentally brushing his ass with her hand, or rather not so accidentally. "I know just what you need Sam."