The restaurant Plavi Podrum is in privileged location, a bit outside the center of Opatija, in a cove for fishermen, a small harbor. It is a very traditional, and while you're some km from Rijeka, the big city. In front you have colored wooden boats for fish and squid. There's no fresher seafood. It is one of the best restaurants in Europe, is in the top 100 according to the magazine Restaurants. It is quite chic. It is good for a nice dinner or a salad at noon. As you near Rijeka, a lovely village, it gets booked on Friday and Saturday night, there are many people. The rest of the time, the service is friendly, beautiful place, and all the attention goes to the little details, either for decoration, cleaning, glasses and silverware, not anywhere. The account is about 40 euros per person for a full dinner.
The Mill is one of the oldest restaurants in Opatija. This small fishing port is located only 10 km from the major city of Rijeka and their especially is grilled fish, vegetables and pasta dishes with a strong Italian influence. The specialty of the region of Istria, are the "tartufe" black truffles, which are often paired with cream and tagliatelle. The Mill offers all these regional dishes along with a wine list that is mostly Italian.
The only drawback to this restaurant is that during the day, there is not much shade on the deck, but once the sun goes down it is the perfect place for a quiet dinner for two, with beautiful harbor views. You can expect to pay about 20 euros per person which includes wine, a fish dish and a dessert. Overall, the prices is reasonable when you know it is one of the most visited cities on the coast for tourists in the summer.