Unable to attack or use hotbar at deep depths - feature or bug?

[Zannyth 2]

So when diving to deep depths (in diving suit - I'm talking where there's no light at all mid day) I have found that I could not use weapons on my hotbar or even punch with my bare hands.  When you're diving to salvage a ship and getting swarmed by predators this can be ... problematic.  Is there a trick to this that I'm not understanding, is it a bug, or is this intended?