Sompasauna - https://www.sompasauna.fi/

Sompasauna is Helsinki's most public sauna - open 24/7/365, run by everyone who uses it. Sompasauna is always free and open to everyone. This is not possible without everyone's contribution - all users participate in keeping the area clean, sawing wood, fetching water, heat up the sauna and keepin the saunas clean.

There is no staff, but the regulars are happy to help if you have any questions or don't know how to contribute. The sauna is run by Sompasauna Association, the area is rented from the City of Helsinki. 

Sompasauna features three saunas all located right next to the sea, so you can jump into the baltic sea for a cool down. There are no showers or changing rooms. If you take your own lock you can use lockers for your stuff. The saunas are mixed gender and swimsuits are totally optional. 

There is a dry toilet in the area and a lost and found box. Campfires at the beach aren't permitted, and there is no option for BBQ. Sompasauna is a sauna place - party, hanging out, drinking and grilling should happen elsewhere. The use of speakers isn't permitted. Clean water for using on stoves and drinking is located 100m from saunas.