And of course you are at least at periscope depth with calm waters? If not try to go a notch higher otherwise you see a black zone through the scope. You can also set 'Stabilized view' to be ON in the career options, this will neglect your sub movements when looking through the optics.
A picture would greatly help.
If you ALT-TAB out of the game you will see grafic glitches and alike. If you do so, then better save/reload. This is maybe the issue here.
XS
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