How much of the ocean has actually been explored?

The answer

Only about 5% of the world's ocean floor has been mapped in high resolution, meaning more of the Martian surface has been charted than the seabed beneath our waves. NOAA estimates that over 80% of the ocean remains unobserved, unmapped, and unexplored. Modern sonar and satellite altimetry have given us a rough picture of ocean depth globally, but detailed bathymetric surveys covering features like seamounts and trenches are still exceedingly rare.