So, seawater density depends on salinity, temperature, and to a lesser extent depth (due to pressure; water is not *quite* incompressible, though close).
It's fairly well-known that fresh water has a maximum density at +4°C, which is why rivers and lakes usually don't freeze all the way to the bottom. This density maximum decreases more rapidly with increasing salinity than the freezing point does, until at about 23 promille, it theoretically lies below the freezing point.
Normal ocean salinity varies from 33 promille (arctic regions) to 36 promille (tropics), the average figure being 34.7 promille. However the Mediterranean Sea (38 promille) and Red Sea (40 promille) are noticeably more saline, while the Black Sea (18 promille) and Baltic Sea (8 promille) are so much less saline that they are officially classed as "brackish", in the same category as river estuaries. This also puts the Baltic Sea at a density profile closer to that of fresh water, including having a density peak somewhere above freezing point (which is why the Baltic freezes over relatively easily, but not deeply).
For a game restricted to the Baltic Sea, the above matters little; I mention it for completeness sake. It's also perhaps worth mentioning that the Baltic has very little tidal activity, though it does sometimes experience local level changes due to storm surges. Tides and currents don't appear in Silent Hunter either, but that's not because they shouldn't be there!
Another significant effect for submarines is that the pressure hull physically contracts under pressure at depth, which is why it creaks when changing depth. This contraction, which could quite easily be measured by the crew, has a significant effect on the sub's submerged buoyancy, and may imply that trim changes must be made if a significant amount of time is to be spent at deep submergence. It was usual to carry a small degree of positive buoyancy for safety reasons, except when actively "hovering" at zero speed, and this will at least partly compensate for the loss of buoyancy due to hull contraction.
As for the maximum diving depth, this was usually quite a lot deeper than the "test depth" officially published. Many designs incorporated a large margin in the pressure hull design strength, so that the sub would have a good chance of surviving a depth charge attack while at test depth. For example, the test depth of British subs was often 300 feet while the actual design depth was 500 feet. Commanders often exploited this difference, preferring to run deeper in order to *avoid* the depth charge attack, rather than counting on the strength of the hull to survive it.
In most of the American "fleet boat" designs, there was a common weakness in that the vent risers for the foremost and aftmost MBTs (located below rather than around the pressure hull) passed through the pressure hull, appearing as pipes (at sea pressure!) within the torpedo rooms. That's probably why you see pipes often being the first point of failure in movies involving a WW2 era sub, requiring frantic damage control activity to stop the resultant flooding. This weakness was corrected in the last of the fleet-boat classes, which however entered service only in the last months of the war, and it seems unlikely that many other navies' designs would have incorporated the same weakness.
|