How many species of plant and animal inhabit the oceans - and how many have we missed?
Bruce Robison of Monterey Bay Aquarium Research Institute said some years back that we were probably missing a third of the large animals in the ocean (he didn't define large) and added, "it could be half." Charles Paxton estimated by species discovery curves that around 47 species two meters long or more might remain to be found. Most of these may be sharks: we keep adding new species to the 350-plus we know of, and an interesting data point is that one reference published in 1986 listed fourteen species known from a single specimen each. This is significant if one remembers that the difference between one specimen and none is usually mere chance. Darren Naish and two co-authors in a 2009 paper in Historical Biology estimated three large, long-necked pinnipeds reported as "sea serpents" could be out there. When I was writing Shadows of Existence (2006), I corresponded with the leading researchers on beaked whales. They were unanimous in believing there were still undescribed species.
The Census of Marine Life tried to count everything - and also tried to estimate what was still being missed, of any size. Examining data including discovery rates and the number of species collected but undescribed in museum collections, the Census came up with 700,000 species known and a million or so total. You can look at the information at the World Register of Marine Species website.
Remember: “The reality is we know more about Mars than we know about the oceans” - Dr. Sylvia Earle