Istria

Look at Poreć, it is very close to Rovinj but not as hilly. Pula could also be a choice. Further up the coast are Novigrad and Umag. A car would be very helpful, Otherwise it is bus travel only.
You’ll find English is widely spoken, as is Italian still.

I have been to Croatia a few times just not Istria yet, and I would suggest two bases--one near the Slovenia border for that excursion, and then somewhere in southern Istria so you can be close to the beaches south of Pula and maybe even take day trips on to the Kvarner islands. If you could stand Dubrovnik in August, you'll likely be fine in Rovinj in August!
If you book early, you should be able to find a place with parking. It really just depends on how lively a place you are looking for.

@Barbara - thank you for the town recommendations - looking at them now!

@valadelphia - Dubrovnik in August was steamy - more reason to swim multiple times per day, and nightly gelato! And yes, trying to get out ahead and book something soon while options are still plentiful.

Take a look at Opatija. It’s built up to be sure, a Central European resort town (in fact the original Hapsburg seaside resort), not to everyone’s taste, not tacky but lively, good swimming without sandy beaches, and very close to Istria. I spent two nights there on a tour, including a day tour to Motovun and other places in Istria, and enjoyed it.

We stayed just outside Rovinj, about a 15 minute walk to town with good parking and easy to access the roads. We were surprised to be underwhelmed by Rovinj but there was a great beach area to spend time at after driving around Istria. There are many beautiful smaller towns. I wouldn't stay in Pula because it's not centrally located, but we didn't get to explore the beach there. English was widely spoken.

Regarding Opatija, I would take a pass. It was lovely when we would visit in the long ago past, but has become very commercial and very expensive. The beach is okay. Every year, depending on the concessions the city doles out, you can have annoying major blow ups (such as slides etc for kids). Also, extremely hard to park unless you book a hotel with a garage.
Also, it is further away from the heart of Istria. In fact, it is not considered part of Istria, check a map.

Am I correct in thinking the eastern coast of Istria might be better for swimming than the western coast -- less open to the sea? If that's the case, I'd give an edge to places on the western side of the peninsula.

I'm not opposed to staying inland, especially if we rent a car. That said, given how much we appreciate swimming, I think it'd be worth it to be able to walk to a beach in 15 minutes or less.

I'm also not opposed to staying just outside of Pula, for example Opatija or even toward Rijeka, if it meant a day drive in/around the peninsula was still easy (which it would seem to be from these spots).

The Mediterranean is generally so calm I wouldn’t sweat the orientation of the beach. In defense of Rovinj, though it is pretty resort-y and busy, it also had imo the best swimming areas I visited. The Golden Cape and especially Red Island were almost exclusively flat rock that dropped nicely into the sea. Tons of fish even at the excellent beach in town with lots of folks casually snorkeling. And again, yeah there’s a lot if people, but it generally meant lots of restaurants. So, if you were going to base here for a stretch you’ve got a lot of options to fall back on. A smaller town might feel a little limiting after a week. You could do worse. That said, I think every suggestion here has its strong points. It’s hard to go too wrong, though I would stay on the coast for certain.

Don’t stay in Rijeka or Opatija. And I say this as a family who has an apt in Rijeka which we visit every year.
You want to be able to walk to the beach early in the morning, do some sightseeing in the afternoon, then have a leisurely dinner in the evening at which ever town you choose.
When we visit we are at the beach around 8-9 am, leave by 11-12, shower, take a drive to a nearby town, and we have our dinner at 3:00. Croatians eat their main meal in the afternoon.
The evening is usually spent with friends or family. You could walk around wherever you are staying and enjoy dessert.

@Barbara - that kind of day sounds lovely! What is it about Rijeka and Opatija that you don't recommend?

Rijeka is not really a tourist town. The hotels are off the Main Street of town and not near a beach. It just doesn’t have that wow factor I would look for when recommending a city. If you aren’t a local you wouldn’t know where to go.
(Thank goodness my husband doesn’t visit this forum cause he would disagree with me since this is where he grew up. I try to explain to him I am looking at the city as a tourist, not a local).
Opatija has been overdeveloped. There are 2 sections in Opatija - the old part with large hotels and beautiful gardens. They also have terraces where years ago there would be music in the evening and dancing. Some evenings there is entertainment but not every night. You can walk along the Lungomare by the water which is peaceful. Some of the hotels have private swimming but they are not what you would consider a beach - more like slabs of concrete with ladders to get into the water.
The other part of Opatija are clubs, casinos, small expensive restaurants, and souvenir shops. When we do go there for a stroll, we stay in the old part.
Opatija is much more expensive than Rovinj or Poreć and other places in Istria.
When we bring friends and family to Croatia, we only spend an evening in Opatija. The bulk of the time we take them to different places in Istria. A favorite is the Roman colosseum in Pula.
With a car you could swim in the morning, drive to a different hill town or city for some sightseeing and lunch/dinner, then back to, say Rovinj, to stroll and have your dessert.
A few of our favorite hill town’s are Grožnjan, Hum, and Oprtalj.

I love visiting both Rijeka and Opatija and I still generally agree with that advice.

Opatija has city gardens which are some of the best in Croatia but it is not really a beach town, it has a long promenade. Germans have also bought up most of the hotels recently and that has made the town change a bit. It would be my personal top choice for a trip to the coast but maybe not anymore.

Rijeka is a great city to visit as well but it is more cultural than tourist focused especially with Opatija being so close.

My advice isn't not to go, because there are destinations for everyone, but if you are flying across the Atlantic for rocky beaches neither of these destinations will do that much for you.

A nearby alternative with beaches which may or may not suit you would be Crikvenica.

Or Krk. Which is mostly European tourists. Cres is a PIA to get to, and I don’t particularly like Otak Krk. Crikvenica is going down the coast, too far away from Istria.
I would stick with Istria.

You're all giving me so much to think about - and so many towns to research - thank you!

I haven’t been to the other places mentioned, but we just got back from Rovinj and it was dreamy. The Golden Cape is perfect for swimming. I wish we’d had two weeks just to ride our bikes there every day, find a spot to plop down and enjoy the stunning view of Old Town Rovinj from across the water!

@Brooke this sounds lovely - we love riding bikes! I'll have to check out this area.

Yes! Just rent bikes right at the main parking lot of Rovinj. They give you a map - easy 15-20 minute pedal to the beaches. There are many options. Worth at least a full day!!