Just by the Marina there is a fenced beach for orthodox Jews. This means during day time it’s separated for men and women, but during the nights there’s usually nobody there and the walls around this beach makes it perfect for a private night swim.
The later you’ll get there, the more privacy you will have. Plus, I don’t know if it’s the walls or divine intervention, but in the summer there’s scarcely any jellyfish there.
Nordau Beach, Tel Aviv-Yafo