What I can tell you is that all versions of Dangerous waters (retail, steam) are moddable since the mods don't touch the.exe file.
As far as which games to buy : well Dangerous Waters is really a quantum leap over 688i hunter killer that you won't believe it.
The graphics is so so, but the simulation and multiplayer/multistation features kick ass.
For the WWII submarine sim you should buy SH III mainly because the game has been patched and has a lot of mods which increase its game value.
At this time SH IV is just a half baked sim with lots of things missing/ broken features etc... so just wait 6 months before buying it.