In Vittoriosa at the Grand Harbour is the Malta Maritime Museum, but it is apparently temporarily closed.

In the National War Museum in Fort St. Elmo in La Valletta is the fuselage of the Gloster Sea Gladiator "Faith", an Italian M1940 Barchino explosive boat, and a few smaller naval exhibits.

In Fort Rinella, also close to the Grand Harbour, there is a 45 cm gun. The same type was used on the Italian battleships of the Caio Duilio class (the older one, not the one in service in WW2). When I was there, below some submarine replicas from the movie U 571 were visible.

Sometimes, warships visit Malta - you have to be lucky...

45 cm L/20 RML 100 t-Armstrong at Fort Rinella

Gloster Sea Gladiator Mk I in National War Museum in Valletta