Croatia is amazing. We stopped at Dubrovnik last year on our Virgin Voyages cruise and had an amazing time there. For that matter, Kotor in neighboring Montenegro was just as good.
My biggest issue with
DCL on these itineraries, having now done both DCL and another line (VV) is that DCL tends to sail out of Civitavecchia. Personally, to me the draw of those Med cruises is Greece, so why not start there. While our DCL cruise was great, it took 2 days to get to/from Greece. Personally, I find it much better to go out of Piraeus (Athens) as you are right there. (Also, I find that for whatever reason, even though Athens is not as "pretty" as Rome, I much prefer it).