The Grave's a Fine and Private Place (Flavia de Luce #9) by Alan Bradley

Flavia is enjoying the summer, spending her days punting along the river with her reluctant family. Languishing in boredom, she drags a slack hand in the water, and catches her fingers in the open mouth of a drowned corpse.

Brought to shore, the dead man is found to be dressed in blue silk with ribbons at the knee, and wearing a single red ballet slipper.