From one of my favorite websites Atlas Obscura:
“… though they’re often used interchangeably, there’s a linguistic difference between the ocean and sea. An ocean is the largest body of water there is, divided into five geographic bodies. Seas are generally parts of these oceans, located where the ocean and land meet. Used in the singular, however, “the sea” is used to mean the continuous body of salt water that covers most of the Earth’s surface—an interconnected global ocean blanketing the planet.”
Who knew?