1. If we are speaking about the ocean as a whole, we might say "the ocean" in the singular. If we wish to emphasize that the body of water is composed of several components, we might say "the oceans," plural. But exceptions abound and often either way is fine; it is best not to get hung up about it.
2. Salinity used to be, and sometimes still is, measured in practical salinity units (psu), which are defined in terms of the electrical conductivity of seawater. Conductivity is easy to measure and a psu is almost equal to a part per thousand, but the psu is not a proper unit of concentration and is awkward to incorporate into an equation of state, and so mass concentration itself is a better unit from a more fundamental point of view.
3. The figure actually shows potential density, which, roughly speaking, is the density with a small correction for the compression by pressure. The data are from the National Oceanographic Data Center's World Ocean Atlas.
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