Using a vehicle with lights underwater (Example: SBU) at night-time, lights on.

Using the forward-facing PIP display, (Hatch irrelevant), white reflections on the edges of objects (Such as rocks) will appear, however general light is somewhat...Non-existing.

Background light (Beyond Viewdistance) is also different in PIP to 'actual' view. The PIP will light the background bluish, and because it is based on viewdistance, will quite effectively outline terrain that must be avoided, lights or no lights.
The 'Actual' view depicts the background as black/completely black, needing lights to see the terrain well.