Welcome aboard demon!
Don't discount the in-game tutorials, or practice missions. The first one is great just to learn and practice what the game subs can do, and what you can do with them. The targeting one is great for learning basic targeting. Use these two generously. After reading/viewing the different tutorials, start a campaign. They seem to be the best overall for learning the game.
As far as mods go, some say install the mods right away, some say wait until you've had some experience. That's up to you. The super mods like TMO and RFB make major changes to the game sub characteristics and enemy AI. I run TMO with RSRDC and other minor mods. And yes, TMO has an environmental mod.
Now stop fooling around. We need you to check the forward hatch for air leaks.