Ditto on what HW3 said about the Generic Mod Enabler. Makes it SOOO much easier to uninstal mods that you try out and don't like.
Here are some I like:
- ail wake mod v2.rar
- ail sky mod v2.rar
- HI_REZ_MOON.zip
- PascalSmoke(1.4ready).7z
These are all just general atmosphere enhancers, all available at the site you mentioned. What they mod, is pretty self-explanatory, I think.
I also like this one:
FABC_Slideout_Mod_14(1.4ready).zip
It's a dials-enhancement slideout ... so you don't always have to ask the engineering station how much air/fuel/battery you have left.
I'd play for a while with some of these before you try out the mods that actually alter gameplay. (Just my opinion.

)
You could also try looking in the "skins" section of Terrapin's site, so your sub doesn't look like a rusty bucket of bolts. Just pick the mod for the sub you use most often.
"FUBARS IXCcamo.rar" for instance, is a camoflage mod for the type IX u-boat. Pretty self-explanatory, for the most part, or you can just ask more questions here.
Have fun.
Did I mention the "Crew Vitamins mod?" Okay, I'll stop. Endless opportunites await.