Was there in June. Really enjoyed it. I did Dubrovnik -> Hvar -> Split, and then back again. Loved Dubrovnik Old Town, and Hvar was also special. Split wasn't that exciting after having experienced Dubrovnik, but it's a good base if you want to explore the surrounding areas (if I had more time, I would have loved to have gone to Plitvice Lakes). I guess it depends on what interests you and your partner.
It's worth mentioning that the larger ferries between the islands are really smooth, so sea sickness is limited (it's just a little rocky when they pull into ports). And if you do start in Dubrovnik and don't want to catch a ferry north, you'll have to either fly to Zagreb and then drive, or drive north but then you'll need another Visa to get through Bosnia since it splits Croatia.
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TPants raised a good point too. Most shops require cash, and while there's tons of ATMs about, many of them charge hefty fees. I also found Croatia to be expensive in general (more expensive than Japan in many ways), so plan accordingly. Uber is available in all of the main cities, and makes getting around much easier (beware of normal taxies, and if you do take one, make sure you get a price upfront).
If there's one thing I'd say not to miss, it's Dubrovnik Old Town. The Old Town can be packed with tourists, but there's plenty of little alley ways to get away from it. If you go, make sure you also take a trip up the nearby mountain that looks down on the city called Srd, either at night or early morning.
