Basically some rope to attach to your sub so you dont get separated or lost, but you could also attach to a item or artifact to trail it behind you then attach the other end to the sub so you dont have to bring that murder in box inside.

The tether could be cut by creatures passing by or if it gets stuck in the engine it quickly reels you in to cut you up if it doesn’t Jam first

Magnetic boots so you can walk on your Sub outside while its moving for body disposal or to take care of some hostile creatures. If the sub, or you gets hit hard enough the Sudden jerk may knock you off and leave you to some horrid fate outside the Sub