No addons can be added to the game by anyone except Vstep the developer for some reason. Many people have made mdoels for it but you have to send it to Vstep and they have to add it into the next SP, if its good enough.
I have SS2008 and also SSE (Ship Simulator Extremes). Cruise ships are available in SSE as are all the ships from 2008 and a few extra ones. There is one warship, a Coast Guard ship and it has a gun but you cannot fire it at anyone. The game is more a simulator than a shoot-em-up war game so the emphasis is on ship driving, towing, mooring, pushing and cargo carrying. I enjoy the odd occasional hour of ship driving and quite like the Dover/Calais ferry run as a free roaming game. Some of the mission scenarios are ok, but some are flawed and unrealistic.