Silent Hunter is a good game, I have it and I have fun with it.
It is very inmersive and have a top graphics, however as "simulation" that is emulating something real and technical, Dangerous Waters is much more simulation.
Silent Hunter III also have a "pathetic" multiplayer, where you can only run subs against dumb AI. Not much fun even using 100% realistic settings
On the other hand Dangerous Waters offer an incredible possibility of multiple plattforms, submarines, Air and surface and the never seen before multistation for a crew to man a single platform.
The only weak point for dangerous Waters are its graphics and that is a more realistic (thus complex) sim which makes it sells less than SHIII and of course on the rankings, selling numbers are important factors, not to mention the "lobbying" of the producers on this case UBI, a gigant compared with Sonalyst, on the area of video games.
For me, although I still play and have fun with SHIII, a really nice game, the best SIMULATION by all means is DANGEROUS WATERS!