Is it warm enough to swim in Split now, September 21st?
On September 9 we were on the island of Hvar, and people were swimming and sunbathing.
also suppose it is what you are used to! I am still swimming in the English Channel. September is always the warmest month for sea swimming as it has had all summer to warm up.
We were there sept 20-22 this year and people were swimming and sunbathing. Weather was beautiful!
As you probably know, sandy beaches in Europe are not found everywhere. There is a very nice one in Split, and it is filled with Croatian folks enjoying the water.
As I recall, the water is pretty nice. September is typically one of the peak months of water temperature in the Mediterranean, warmer than June. Do consider some water shoes, most beaches are pebbles rather than sand.