If you are going to do the city walls (which you of course should), suggest you buy the 1 day Dubrovnik card for 130 kuna per person. Walls are 70 each, throw in this museum for another 40, then you can just hit one other and make it a good deal (i.e. the Rector's palace or the Ethnographic museum). The Maritime Museum (which is in St John's Fortress) has an interesting but digestible history of the maritime trade in Dubrovnik. I'm not that into maritime stuff but still found it interesting. Takes about 30-40 minutes to read the main plaques and check out the models, etc. Also, it's a good stop along your walk of the walls (at the half way point).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.