The memo had 'have them practice floating in water' scrawled on it.
Literal
Memo [in-には], water [in-に] float practice [object-を] make-do [goal-ように] [quotative-と] scribbled was.
殴り書き ('scrawled writing') — literally 'hit-writing' — describes hasty, rough handwriting. させる (causative of する) here means 'to make someone do' — the memo instructs that someone be made to practice. 浮かぶ練習をさせる = 'make them do floating practice.' The passive されていた ('had been scrawled') sets the scene.