I've been on the phone with the devs and they pointed out a lot of ways game's file structure is open to modding. Boys, this game is seriously moddable.
You can create your own missions, and your own campaign even, but you can also customize the sounds you associate with different events and pieces of equipment; you can change the HUD for a single sub if you want (ex: you could make a HUD for the Permit class boat that appears different that the Skipkack), so when you get a sub, it has its own customized feel.
Of course, you can tweak the performance attributes of the ships and aircraft, so if you want the Alfa to be a little noisier than it is in the game (and doesn't everyone love a noisy Alfa?), that's possible.
And yes, the missions have an amazing amount of RANDOM factors you can use, so replaying a single mission will never be canned. I especially like that.
Dr. Paul has posted a
Modding Guide here. It will be updated and expanded on as time passes.
So, yeah, not only will you get a lot of enjoyment from Cold Waters straight out of the box, there are a lot of ways you can get more gameplay from the upcoming mods that Subsim mod gurus love to create.
