Yes, "Marinegeschütz" ("gun of the navy") seems correct, another term would be "Deckgeschütz" ("deck gun"). Marinegeschütz is a bit more official (perfect for the used official military letter), colloquially just "Geschütz" ("gun") or maybe "Kanone" ("cannon") would be used.
As always thank you for the great dev log – your attention to detail is awesome!
I wish you a speedy recovery!