In Istria the only beach we visited was in Pula. It was fine, nothing special.
Don’t bother with the beach in Split, it is way too small and crowded. We’ve never been to the beach in Zadar. In Dubrovnik we swim in 3 different places.
1. Banje beach, but we are there by 7:30 and leave by 9:00. No crowds. It is a short walk if you are staying within the walls, which I highly recommend. It’s so easy to get there early and many locals swim there, I know our cousins do.
2. We take the ferry to Otak Lokrum and swim on the far side of the island. There was a nude beach also but we never went. Don’t even know if it’s still there. The ferry cost is pretty steep but we usually spend the day there. The beach is never crowded, mostly local pensioners, they have a ferry pass.
There is one cafe there where we eat lunch. The island is a National park so no one lives there. If you get an early ferry, just follow the old folks with swim gear.
3. We sometimes swim off the pier around from the aquarium. They have ladders to get in and out of the water. For a quick late afternoon dip this is where we go.
Remember that the beaches are all pebbles so water shoes are necessary if, like me, you have delicate feet, lol.