Of the more than 500 or so shark species, about 80% grow to less than 1.6 m and are unable to hurt people or rarely encounter people. Only 32 species have been documented in biting humans, and an additional 36 species are considered potentially dangerous.
Almost any shark 1.8 m or longer is a potential danger, but three species have been identified repeatedly in fatal bites: great whites, tigers, and bull sharks. All three are found worldwide, reach large sizes, and eat large prey such as marine mammals or sea turtles. More bites on swimmers, free divers, scuba divers, surfers, and boats have been reported for the great white shark than for any other species. However, some 80% of all shark bites probably occur in the tropics and subtropics, where other shark species dominate and great white sharks are relatively rare.
An estimated 50-80% of all life on earth is found under the ocean surface and the oceans contain 99% of the living space on the planet. Less than 10% of that space has been explored by humans. 85% of the area and 90% of the volume constitute the dark, cold environment we call the deep sea. The average depth of the ocean is 3,795 m. The average height of the land is 840 m.
"Currently, scientists have named and successfully classified around 1.5 million species. It is estimated that there are as little as 2 million to as many as 50 million more species that have not yet been found and/or have been incorrectly classified."
According to the World Register of Marine Species (WoRMS), there are currently at least 226,408 named marine species (9/24/2014).
So, there are at least 226,408 marine species but there are most likely at least 750,000 marine species (50% of 1.5 million species) and possibly as many as 25 million marine species (50% of 50 million species).
The oceans cover 71% (and rising) of the Earth's surface and contain 97% of the Earth's water. Less than 1% is freshwater, and 2-3% is contained in glaciers and ice caps (and is decreasing).
90% of all volcanic activity occurs in the oceans.