Yes, Tom is right DW is really the sequel to 688/Sub Command it really is not like Fleet Command at all. It is definitely much more of a simulation than than the RTS style play of Fleet Command.
Sonalyts description above is a bit misleading... "allowing you total control over multiple air, surface, and submarine platforms in a modern-day naval environment!"
Whilst it is true there are multiple units in the game you only command one unit at a time per game, sim style as in 688/SC (at the same level of "simulation depth" as those games) not multiple units and you can not jump between different units The only time you control more than one unit is the frigate/heli combo and then it's only by remote control via ASTAC
DW is definitely what you are looking for judging by your description although the graphics are not quite DX10. A
Demo is available , but the whole game is now very inexpensive and definitely worth buying. I think some printed manuals are still available via Neal at Subsim.com they are definitely worth getting as they are huge!
The multiplayer though little used sounds great too, as apparently different players can man individual stations on board the same sub something simmers have long requested.
I think realistically DW may be the last modern subsim as it didn't sell very well for Sonalyts (despite their heroic attempts at marketing it) and things have gone very quiet from them recently.
Whilst some at Subsim wish for a modern Subsim from Ubi the ground swell currently is more in favour of a WWII surface sim sequel in the mould of "Destroyer Command" or the even more popular old Microprose classic "Task Force 1942" which also let players command cruisers and battleships.