If you want the wonderful atmosphere of WWII subsimming, then the Atlantic based Silent Hunter 3 with the GWX mod is an excellent choice. Silent Hunter 4 is superior in terms of its graphical appearence, and it has more gameplay elements to offer. With additional patches and mods over the coming months, SH4 will be awesome, but for real atmosphere, the U Boat based SH3 GWX would be a wonderful introduction for you.
