Answer: It would be ocean's
Explanation: In this context the apostrophe show ownership, or WHOSE waters became choppy (I know an ocean is a WHAT and not a WHO but phrasing it like this in my mind always helped me). Without the apostrophe it would be the plural form of OCEAN.
Expository paragraphs should be clear and be concise. This is because they should be brief, get to the point and explain the point, and then wrap up.