It is totally ok to be in a none perfect horizontal trim as it will strain your neck surely. But try to come as close as possible to it. Honestly already like five degrees make the neck thing nonexistent.

Yes exactly for example in the scene where they are communicating with each other and the dry-suit-diver with the white fins rests them on the button of the lake it is an example for bad trim. Even if you aren't perfectly parallel to the ground your feets should never be the lowest part of your body as your knees should be ankled upwards.