On a budget, weigh it — Maritime Museum (Fort St John) isn't cheap for what it is.
~€10 (often on museums pass)
Come if you want the answer to 'how did a city smaller than a town out-trade Venice?' — by the 16th century Ragusa ran one of the Mediterranean's largest merchant fleets, and the word 'argosy' for a great trading ship comes straight from Ragusa's name. The ship models, charts and the story of insurance and consular networks make that empire tangible, all inside the harbour-guarding Fort St John. The honest catch is it's a traditional, text-heavy hang, and the fort's harbour views are nearly as good a reason to climb up as the exhibits. Best for the genuinely trade- and sea-curious.