The only advantage seawater usually has is it’s ’free’ if you’re on the coast. But in many habitable climates, freshwater is ‘free’ too. Like most of Finland.

Finland has vast stores of fresh water and annual precipitation has nothing to do with it.

Or is it the the sun and rain that created the lakes?

Thanks to the last ice age and retreating glaciers...

The fact that they are still full of water is due to them being replenished by precipitation more than they lose water. If they would lose more water then what flows into them we would be speaking about the "land of many bone dry holes" in the famous Finnish desert.

Fresh water makes the entire project much cheaper.

Don’t want to kill all those underground pipes with mineral buildup as things precipitate out as it cools.

The water pumping and maintenance costs are likely going to be their largest ongoing costs, no?

No, the only cost really is the cost of heat exchanger and maybe energy to pump water through the heat exchanger. Heat exchange approaches 100% thermal efficiency with large enough heat exchanger.

Not really, heat pumps as a transfer method can achieve better total efficiency and make distribution of the heat easier.

They’re looking to make cheap use of the existing waste heat, after all, which while ‘low quality’ is high enough quality for district heating already.

https://www.slideshare.net/HelsinginEnergia/innovation-and-s... (slide 21)

They mention they are recovering heat from waste water. Obviously, they aren't pumping waste water into the system but rather use heat exchange to extract energy.

My favorite so far has been Vienna - I would describe it as similar to Evian.