The two, IMHO...VERY essential game-play and realism modifications to recommend would be...
1.
Natural Sinking Mechanics 2.6 ....by WernerSobe
and...
2. New, Improved Campaign Layers version 0.73.... by tater
These are essential..but there are others, and you'll find them, soon enough.
And ...when you get around to caring what the ocean looks like...
you MUST have
.....Living Breathing Ocean mod.....by
leovampire
All found here...
http://www.subsim.com/radioroom/showthread.php?t=118945
Cheers!