On the surface he was calm, but inside he was churning.
Tips
history
Recorded in 《三国志·吴书》 commentary: Cao Pi of Wei, standing on the north bank at Guangling, saw the Yangtze "with its waves surging and crashing" and sighed that heaven itself had divided north from south.
usage
Often literal for seas/rivers, and very common in figurative use for inner turmoil — 内心波涛汹涌 is a writing cliché for hidden emotion.