She dove beneath the surface of the water.

Literal

She [topic-は] water-surface below [to-に] dove.

水面下 (すいめんか) means 'below the water surface' — 水面 (water surface) + 下 (below). もぐる (潜る) means 'to dive' or 'to go underwater.' 水面下 is also used figuratively: 水面下で交渉する ('negotiate behind the scenes').