can i ask why is it so important to have a realistic flight model?
Because if you don't, then any underwater mission is just spaceflight in blue space.
and FS is an arcade shooter in space and is one of many games that prove "realism" isn't necessary to be fun. OK so it would be FS with blue backgrounds, guess what if the story and the missions are executed well it will be a far superior mod to one with an atmospheric/aquatic modeling system and only mediocre story/mission design. yes these features would be cool, yes they would greatly expand on the engines applications, yes i would like to see them but frankly they are a nicety not a requirement
The main issue I would have with that that there would not be any "upright" position and the peculiarities of submarine combat would be largely unappreciated.
With a hydrodynamic physics model, this would largely be solved automatically, as the players (and AI when done right) would need to stay in upright position while traveling straight and level.
Secondly, calculating pressure would be trivial if you had a surface (and overlaying barometric pressure) on the ocean to start with. Below certain depth, ships' hulls would start taking damage. This would allow an unseen tactical perspective - ships with stronger hulls could go deeper than ships with lighter hulls. Submarines can't dive as deep as you might think. Take for example the large nuclear submarines - Ohio class has test depth of 240 metres, while its entire length is 170 metres. Akula class has test depth of 400 metres and length of 175 metres, German Type VII U-Boot from WW2 had test depth of 230 metres and calculated crush depth of 250-295 metres and total length of 67.1 metres.
So, in most environments, the bottom isn't so much of importance, but surface most certainly is.
Also, since a lot of submarine warfare used to be directed against surface vessels, you would lose a lot of that if there was no way to model the surface, and surface vessels accordingly.
Then there's the matter of detection systems. Active detection paints you as a target, while passive detection would basically mean listening noises carefully, and that would depend on how fast the targets are moving and how much noise they make while doing it. That would, technically, be doable with radar with blipping dots, detection distances etc.
Then there's the matter of boundary layers which cause problems to detection.
And what would you use as weapons? Torpedoes? With automatically locking torpedoes in FS2 style, that would very fast end up in repetitive and boring gameplay. Projectile weapons, under water? Blegh. Beams as lasers? Better but still unplausible.
As a whole, I think there are ways to cleverly mimic some of these things in FS2_Open engine, but it is not very well suited for this kind of thing.
Besides most of submarine combat happens BVR, and there aren't really any observation windows or cameras to be used either - it's done with just sonar data. If you wanted to do something in shallow enough water that sunlight actually illuminates things, then you would need to deal with stuff like how the surface waves affect the lighting, how currents move things, etc. etc. And modeling of marine animals...?
