Nicely detailed model
I will asnswer your question with another question: were those guns actually covered?
Serviced and greased whenever possible yes, antirust coating being re-applied at the first sign of deterioration, yes, barrel closed with a plug when the gun had not to be fired, yes, but covered with a watertight coat... mmm, I doubt that. Putting in place and/or removing such a cover might have meant a waste of time in circumstances where a crash dive or a swift gun attack were needed. Not to mention the fact that the extra drag caused by a cover or, even worst, the retention of air under it, might have affected negatively the handling of the submarine once submerged.