This subjct is not something to worry about, it is all a very minor effect, and of no consequence in calculating deco using a computer. As mentioned above, the computers measure pressure, and that is all that matters (except time, of course, and those few that want to know water temp as well). The only time the salinity comes into play is in the conversion from pressure to depth. Even then, salinity has a minor effect. Over the range from fresh water to the saltiest water, it is only a variation of a few percent in the pressure-depth equation...typically at most 3% in the ocean. It turns out that temperature has a much larger effect! Cold water is denser (heavier) than warm water, so other things being equal, warm salty water will float on cold, fresh water. The point is that sea water density is what is important, not salinity per se, and the density does not vary much. Fresh water has a density of 1 (gm per cubic centimeter) in contrast to about 1.025 at the ocean surface. Really salty, really cold water on the bottom of the ocean has a density of 1.05....only 5% heavier than freshwater. And much of that increased density is due to the weight of 600 atmospheres squeezing down the water....water is actually compressible and does so at the bottom of the ocean. Without the pressure effect, the water density at the bottom would less than 1.03.
So, over the range of locations (fresh to the Red Sea) and depths (less than 1000m) and temperatures (40C to -1C) in which we dive, salinity and temperature and depth only change the pressure-depth relationship at most 3%. And the computers do just fine working on just pressure. And that is why we call 1 atmosphere 33 feet in salt water and 34 feet in fesh water.....3% more.
Thus endeth Physical Oceanography 101. Happy New Year.
