Short answers, pulled from the story.
The sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. It covers approximately 71 percent of the Earth's surface and holds about 97.2 percent of the planet's known water. Earth is the only known planet with seas of liquid water on its surface.
The sea is salty because of dissolved solids, the most abundant being sodium chloride, the same compound as table salt. The open ocean holds about 35 grams of solids per litre, a salinity of 35 parts per thousand. The Mediterranean Sea reaches 38 parts per thousand and the northern Red Sea can reach 41, while the landlocked Dead Sea holds 300 grams per litre.
The deepest part of the sea is the Mariana Trench in the West Pacific, near the Mariana Islands. Its deepest point lies 10.994 kilometres, nearly seven miles, below the surface. The trench extends about 2,500 kilometres across the seabed.
The global conveyor belt is the thermohaline circulation, a deep ocean current driven by differences in water density caused by salinity and temperature. Cold salty water sinks near Greenland, flows south through the Atlantic, joins more sinking water near Antarctica, then splits into streams moving north into the Indian and Pacific Oceans. The full circulation pattern takes a thousand years to complete.
Tides are caused by the gravitational influences of the Moon and the Sun and the effects of the Earth's rotation. The Moon is some 27 million times less massive than the Sun but is 400 times closer to Earth, so it has more than twice as great an effect on tides as the Sun. Most places experience two high tides each day at intervals of about 12 hours and 25 minutes.
Most ocean waves are less than 3 metres high, and strong storms can double or triple that height. Rogue waves, caused by constructive interference between waves from different directions, have been documented at heights above 25 metres. A wave represents a transfer of energy across the surface rather than a horizontal movement of water.
The Challenger expedition of 1872 to 1876 effectively created the science of oceanography. On its 68,890-nautical-mile journey around the globe, HMS Challenger discovered about 4,700 new marine species and made 492 deep-sea soundings. Earlier, scientific oceanography drew on the voyages of Captain James Cook from 1768 to 1779.