(http://xhatahx.weebly.com/uploads/6/0/1/8/6018829/screenshot-doom-20161002-155821_orig.png)
This is the "front" of the submarine (you can see eyes there). As you can see, it's impossible to climb it up. If you look behind it...
(http://xhatahx.weebly.com/uploads/6/0/1/8/6018829/screenshot-doom-20161002-160338_orig.png)
...you can see you can climb it up. I suggest making climbable ledges on this side to make a new route for players to got to, thereby making the map (slightly) easier to navigate, and making the Super Arm easier to collect.
The Super Arm there just feels like it's taunting you with the fact that you have to go to the other side to get it.