Well,
I believe that we are "aware" of hundreds if not thousands of species. The key factor is that scientists have not identified and classified them yet. They are probably reasonably commonplace to locals living in remote areas, but field trips there are not to pleasant and are costly.
As for the oceans, we really have no idea, as we have enormous access problems. I would remind you of the coeleacanth. We thought it had been extinct for 60 million years until someone found out (by chance) that fishermen in Madagascar were still catching them :D
