In as much as a gun on a submarine makes sense then an aft mount makes as much sense as one mounted forward.
Fleet boats had their conning towers relatively farther forward than U-Boats. This left more space aft.
Spray interference from the bow would not affect an aft mounted gun.
What's the gun for? With complete firepower superiority it matters not at all where the gun is mounted since the sub has freedom of maneuver. Without total firepower superiority you have no reason to voluntarily use a gun from a vessel as vulnerable as a submarine.
If you have to escape on the surface and under fire, the aft gun mount simplifies and improves your course options as its "A" arc will be towards the enemy as you flee.
Most Fleet Boat captains did fine with aft mounted guns although many never actually used them on patrol. I would suggest that gamers love deck guns far more than real life skippers ever did.
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