Bačvice: just east of the Riva. Usually very busy.
Žnjan: in the southeast, it’s a long beach that’s only used for swimming and sunning.
Kaštelet and Kašuni: both of these are on the Marjan peninsula and are especially beautiful and wild.
South to Omiš: there are beaches in every village on this stretch of the coast and just off the road. These are highly recommended options for your swimming, especially if you don’t like having a lot of humans all over the place. The road trip can be frustrating however for its great traffic.
North: easily to travel to these roadside beaches via car, they’re all over the place and make for great swimming.