Good eye! On shore there is a second conning tower from a WWII era sub, for kids to climb on. There is also a complete torpedo tube on display. The entire tube has been removed from the boat, and you can see all of the 'guts.' There are also a couple of torpedoes on static display, and a prop or two.
However, the item you were asking about is a diving bell. It is up on stilts so you can look into the bottom of the bell, and kids can crawl up inside of it as well. From that photo it sort of looks like some Jules Verne Moon Lander.