To quote the National Ocean Service:

Yet for all of our reliance on the ocean, 95 percent of this realm remains unexplored, unseen by human eyes.

NOAA’s Office of Ocean Exploration and Research is leading efforts to explore the ocean by supporting expeditions to investigate and document unknown and poorly known areas of the ocean. These expeditions represent a bold and innovative approach by infusing teams of scientist-explorers with a "Lewis and Clark" spirit of discovery and equipping them with the latest exploration tools.

From mapping and describing the physical, biological, geological, chemical, and archaeological aspects of the ocean to understanding ocean dynamics, developing new technologies, and helping us all unlock the secrets of the ocean, NOAA is working to increase our understanding of the ocean realm.

In general water is an obstacle for satellites mapping deep ocean ground.