This is a great place to learn about the history of the US and the 2nd World War. Walking through the museum is free, there is an entrance fee - with discounts for minors, seniors and military personnel - if you want to board and walk through the submarine or any other warships docked at the pier.I found it to be highly worthwhile to check out the ships, its a unique experience. The photo is of the 20mm twin anti-aircraft machine gun mounted on the submarine.
Historic Pier 45: The Triangle is a free museum and there is an impressive machine gun inside.