You are correct in that it's not very useful. Only the KM SS4 can really do interesting things with its guns.

As to what to put there, Engi and Seaman need to be in a support slot to work, but a repairer or restorer will give you full benefits anywhere, so those (or just leaving it empty to save weight) are the most common choices.

Thank you. Very helpful!

The sub gun can be useful if you are chasing a cv on the surface.  Also once in a sub v sub duel we circled each other until we both ran out of air but I was able to sink my opponent on the surface using my deck gun as we continued to circle.

So you suggest to put gunner in SS 5/ 5.5 and 6