Yup... that's the game. Of course, if you have early war paint, where your boat is all dark black, then your sub looks like a shadow against the little ambient light there is, especially when viewed against the ocean reflectivity... If you're handy with the Mission Editor, you could build yourself some "test" missions with enemy surface ships approaching each other under both daylight and night time conditions, and see how much of a difference there is, paying attention to their range from each other when they start shooting. There is always a trade-off in trying to balance night vision versus day vision. Then do one with an anchored enemy ship, and approach it with your submarine from various angles and see what you see (or don't) happens. Use of the Save with Replay can help when reviewing test missions.
