In a Subal or Nauticam housing I use Nauticam viewfinders. Both have advantages and you really need both.
The straight thru (gunshot) is great for finding fish on the move or also a night dives to locate things faster.
The 45 is my go to in most daytime situations shooting macro for example. You can get on the same eye level as subject and it easier to frame the view from above looking at subjects.I
Its not one or the other its both are needed for the best results for me..