(Another long one. Water is...just so important. The basics never stop mattering.)
As inferred by the name "thermohaline" with respect to density-driven currents in the ocean, the main properties of ocean water we measure are temperature and salinity. Functionally salinity means conductivity, because the salt content of ocean water is related directly to how electrically conductive it is, and that is easy to measure. One of the most basic tools for oceanographers is the CTD, short for Conductivity-Temperature-Depth.
In research the CTD is used to infer density. In applied science it is used more often to determine the speed of sound in water, for use in acoustical applications. Electrical conductivity is measured as a proxy for salinity. Temperature is measured directly, and depth is inferred from water pressure. In this way scientists (both applied, as I am, and researchers) can obtain a vertical set of measurements through the water (through the "water column", a vertical section from sea surface to sea floor).
Because water is a polar molecule, salt dissolves readily in it. About 85% of salt in the ocean is
sodium chloride, which we know as table salt. Since salt molecules are bonded ionically--the sodium (Na+) atom surrenders an electron to the chlorine (Cl-)--water easily separates them and carries them in solution. (This is why hydrocarbons like gasoline and oil do not dissolve in water: hydrocarbons are not polar, so they are not sensitive to water's hydrogen bonding and instead stratify according to density.) The sources of ocean salt are thought to be eroded continental rock and volcanic outgassing (including discharges from hydrothermal vents, which are heated by magma below the sea floor).
Over billions of years, with the constant cycling of water around the surface of the planet, the ionic content of the oceans has only increased, because salt ions do not evaporate. For this reason, at any given time, salt content of the ocean is considered constant. The total salt content of the ocean is estimated to be 1.4*10^20 g, or 1.4 sextillion grams. The mean salinity of the ocean is 35 parts per thousand (ppt), or physically, 35 g salt/1000 g water. An alternate measure is the practical salinity unit, or PSU, which is simply ppt - 1000. (1035 ppt = 35 PSU.)
Ocean salinity is not constant around the world. It is lower where there is an influx of fresh water. For example, rain, a regular occurrence in the northeast Pacific, produces a comparatively fresh layer on the surface of the ocean, as does the discharge from large rivers like the Nile or Amazon. And where evaporation dominates, like in the eastern Mediterranean or Red Sea (or in extreme cases like the landlocked Dead Sea), salinity is higher.
The ocean's mean density is 1029 grams per cubic centimeter (g/cm^3), rather more than pure water (1000 g/cm^3). The density changes with temperature. Seawater is at its densest around 4 deg C (39 deg F). It expands as it gets colder from there, with ice being about 90% as dense as water (why 9/10 of icebergs are below the surface). Water also expands as it gets warmer, which is why the ocean stratifies as it does with the warmest water on top, and the coldest on the bottom.
A basic means of representing the state of water column at a location is the Temperature-Salinity Diagram, or TS diagram for short. A vertical section of water is profiled with a CTD and the results are plotted with temperature on the vertical axis, salinity on the horizontal. The diagram can be used in a number of ways, but the two most common are to plot a given water mass as a region, showing the temperature and salinity properties common to that type of water. In the first figure, North Atlantic Central Surface Water = NACSW, Mediterranean Intermediate Water = MIW, Antarctic Intermediate Water = AAIW, North Atlantic Deep Water = NADW, and Antarctic Bottom Water = AABW. Another method is to profile the CTD cast as a point plot, showing the changing properties of the water column with depth (which can be roughly inferred from temperature). In the second figure, the surface water smoothly blends into the intermediate water. The thermocline is more of a halocline--temperature is consistent while salinity and density increase dramatically--followed by colder, saltier bottom water. Plots from around the world will show vastly different water properties at different depths.
Tomorrow: dissolved oxygen.
Temperature-Salinity Diagram showing some particular water masses.
Temperature-Salinity diagram plotting one CTD cast.
Be well!