Yeah, it's definitely more of a physical oceanography thing, and even then it's sort of niche because there are only certain situations where it's more useful than just using density

Here's a paper from just this year that uses spice. The GSW package included with TEOS-10 includes functions for calculating spiciness. It's definitely a term that gets used. Here's the paper that gives the mathematical definition of spiciness that's used in TEOS-10. It is different from density in certain ways.

To add onto this about the pressures that exist in the ocean, pressure at any given depth is equal to the density of water * g * h. Let's say we have a water column consisting exclusively of pretty dense water with a density of 1029 kg/m^3. To get 1GPa, the ocean would have to be 99 km deep which is far far deeper than the deepest part of the ocean.

The density of seawater is affected by both temperature and salinity. The exact relationship is pretty complicated, but fresher water can be denser than more saline water if it's considerably colder. That's why physical oceanography has a concept of spiciness. Warm and salty water is 'spicy' while cold and fresher water is 'minty.' Because density is affected by both salinity and temperature, minty and spicy water can potentially have exactly the same density.

Here are a couple of figures I pulled from Talley's Descriptive Physical Oceanography textbook. Here's a map showing the temperature and the salinity of the circumpolar deep water around Antarctica. If you compare it to the Antarctic bottom water, you can see that the Antarctic bottom water is a little fresher than the CDW, but it's considerably colder.