Hello...

[Blacklight]

To halfway answer your question, you do not "need" to install GameShadow, or whatever SH3 uses. I purchased SH3 from the store, and when installing, it asks to install GameShadow, I click cancel, because I update manually. The game still runs 100% with nothing stopping it.

I actually find Gameshadow a pretty decent and helpful little program. It keeps all my drivers and a good chunk of my games up to date without me having to check for updates constantly.
If you want to install the GWX Mod (and who wouldn't want that ?), don't install the one that Gameshadow has. They have an older version than what's current. Get the latest version here:

http://gwx.mysite.orange.co.uk/

I also suggest the Torpedo Damage Final version 2.0 mod that makes torpedo and deck gun holes in ships that actually LOOK like torpedo and deck gun holes. Get that here: http://hosted.filefront.com/OceanBlue

I also suggest getting the RealweatherFix2 Mod that seems to help a little with "weather issues" the game gets (Make sure to follow the instructions before installing it)
http://hosted.filefront.com/TeddyBar/

Also, Don't ship out without Silent Hunter III Commander (If you use GWX, make sure you read the manual part pertaining to SHIII Commander)
http://www.users.on.net/~jscones/software/products.html

I suggest updating the game to version 1.4 and playing it stock without mods for a little while first so you can REALLY see the difference that these mods and addons make but keep these links handy !!