Currently I'm using Control Keys 1.1, which claims to have moved the screen grab to PrintScreen, but hasn't really.
First you need to hide the UI (
Shift-Z or
Numpad Del for me), since you can't once the game is paused, then I press
Backspace to pause the game. There's no "Game Paused" marker though. Then fly wherever you want to take your picture and use
Ctrl-F11 to take the shot. One interesting thing is while paused the waves will change with your camera angle.
If you're in question, alt tab out real quick and check that it created the BMP file in
Ubisoft\Silent Hunter 5\