There are two possible answers if someone asks "where I can swim in Split". The short one is "Wherever you can enter sea water, except in city harbour and around shipyard". The long one would include explanation that it's pretty unique for city of 200,000 to have so many attractive swimming spots, with water clear almost like somewhere on islands. In short, almost whole water line on Split peninsula is suitable for swimming, with a great diversity ranging from popular sandy beaches, to pebbled beaches, to wild rocky places, especially at Marjan Hill shores.
These five beaches - sorted from East to West - are obvious choice, not only for sun and sea, but for something more they can offer. Locals love them, although we have our own local favourites which tourists might not like, usually because of same reasons why locals love them. For start, here's the map with these five spots. Of course, you can explore by yourself, to find a place that will suits you the best.
1 Trstenik beach
Being maintained by the Radisson Blu Resort Split this is probably the best kept beach in Split. This pebbled beach is not completely natural, but is well equipped with sunbeds and parasols. Still, there are not too many of them, so you can easily come there and put your towel wherever you want. There is a VIP platform operated by the hotel, but the rest of the beach is completely open for public. Beach is clean, with some bars around. Water is very clear, mostly because it's open for currents, and if you need bigger company, over crowded Žnjan is just around the corner.
If you want privacy at the beach, avoid Bačvice (pron. Batchweetseh). If you want to meet people, this is the first choice. It's the central city beach, popular among locals and young tourists looking for some fun. It's a sandy beach, with sunbeds and parasols (maybe too many of them, but they are usually all taken). For people in Split, Bačvice is part of their city's identity, and if you see some guys (and sometime girls) running around and jumping while trying to hit small ball with a palm, it's picigin. This beach game was born here, because solid sandy seabed is perfect for short sprints and parades. It's also probably the only beach game protected as a cultural heritage. Bačvice lives 24/7 during the season, because many bars and clubs are one of the biggest nightlife zones in town, including Split Beach Festival. Sea water is surprisingly clear, for beach with so many people, except later in the afternoon.
Only about twenty minute seaside walking from the city centre, you will reach pebbled beach just below the Ivan Meštrović Gallery, and southern slopes of Marjan Hill. Main population on this beach is twenty-something crowd, families are usually based little bit away. Water is clear, it's not populated as Bačvice, and there are bars right on the beach if your really can't stand being thirsty. Of course, if you don't like loud music while swimming you will go somewhere else. This is also good spot to meet local people.
It might look difficult to pronounce, but here it is: "kahshyounee". It's another Marjan beach, with position close to perfect. You can reach it on foot (not recommended in the middle of summer), by public bus number 12, by car, by bicycle. You can come there straight from town, or from the Marjan Hill, as a refreshment after hiking or jogging, or whatever you do for exercise. But what I love the most about this beach is view from the sea. Get in the water, look up, and you will see Marjan Hill in its full beauty, with pine forest, churches, rocks, etc. Water is fine, especially if you swimm away from the beach. Recently, this beach god its concession holder, meaning there are some beach amenities, like lounge bar or seabeds, and more is yet to come. Very close is also the Split's only, and unofficial nude beach.
We are still at Marjan Hill, on the beach which is part of recreational centre in the pine forest at the northwestern part of this popular forest park. There are several tennis courts, football (or soccer) field, playgrounds for children, and even open-air gym. Also, there are kayak tours starting from that spot. You can reach it by bus, but also walk through the woods, or rent a bike and go there like most locals do. Bene is popular among families, especially with children. Water is not as clear like on other beaches in this selection, but mostly because of the sandy sea-bed, not because it's polluted in any way. There is a bar and a restaurant right next to the beach, and in case you like some privacy, area around Bene - named after church of Saint Benedict that once stood there - is full of solitary spots.