Apropos: Helsinki has never experienced heatwave longer than 5 days (above 25°C). Last summer it was phucking 45 days.

However the bottom temperature of the near-by Baltic Sea is -4°C. I know this because I was hunting for halibut.

There is already a separate system which refrigerates government offices from the Baltic Sea.

I would suggest is to use the system to cool all homes by using the central heating system. All you need is lower is temperature to 15°C in those pipes.