That's the museum inside the Navy Yard at La Spezia. Access is on the side of the main entrance, towards the far left of this picture.
It is not the most active museum but there is a fair amount of interesting kit and fare is basically symbolic (less than a couple of € last time I was there). Among the highlights a side by side
exposition of WW1 and WW2 era manned torpedoes, one of the
container used for the 1941 raid on Alexandria and recovered from the sunken Scirč, a collection of 19th and 20th century small to medium caliber naval artillery (such as
this WW2 era submarine gun) . My favorite piece is the mini torpedo boat:
If you are interested in more modern warships there is also a beach shipyard not very far at Riva Trigoso
