How much of the sea has been explored?

It's common to read figures like "95 percent of the ocean is unexplored." Is that right? What does it even mean?
Well, "the ocean" is millions of cubic kilometers that is always in motion.  It doesn't stand still to be explored, but we're mapping currents and zones and thermoclines and stuff, so we're doing our best. But the only place you could make any kind of percentage assessment is on the seafloor.
Dr. Craig McClain in  this source tries to figure it out for the deep seabed.  He takes the deep sea as meaning the areas past the relatively well-known continental shelves, so we're talking about the bottom under water 200m or more deep, and that works out to 106 million square kilometers. His rough estimate of how much we've sampled using ROVs, submersibles, cores, and trawls (as opposed to just mapping contours with sonar, an important effort but one that misses a lot of detail), is about 1650 square km.  That works out to an explored area of 0.0016 percent.
So you could say the seafloor is well over 99.9 percent unexplored.

We've got a lot of work to do.