Why locals love it?
Being beachside in Helsinki is not actually as bad as it might sound and during the loooong summer days you can spend many pleasant hours between the sea and the sand. There's also nothing to fear - not much lives in this brackish sea except a few brave, yet toothless fish, and the waters are pretty reliably calm and unchanging.
Why you should visit it
The Copacabana of Helsinki. Go hang with the beautiful people at ‘Hietsu’ if you want to experience some sand, some sun, and the tropical charms of the Baltic. Swim out to the small island not far off the coast and explore the seabirds’ favourite hangout.
If you don’t see the locals swimming then check the info boards or ask someone if there’s a lot of "sinilevä" - blue-green algae afloat.