It's not advisable to enter a gunfight against an armed vessel. I was once sunk by a Tramp steamer. Don't ask...
In my opinion, the merchants are far too deadly. Once or twice, I torpedoed a ship on surface at night, and once the ship was hit, they immediately pointed the searchlights at me, and start shooting. While I was diving, they scored a several hits. Although they were in flames, sinking.