"The waterways of Stockholm are not only beautiful to look at, they are so clean and clear you can take a dip right in the middle of town. There are not a lot of cities in the world that can make that claim!"
Source

Stockholm is also so clean you can even go fishing in the water
As pointed out in the SAVE MONEY section
""...You may also fish in the capital Stockholm that has so clean rivers you may even swim in them...The waters of downtown Stockholm are teeming with premium quality salmon and sea trout. As you pass by the Swedish Riksdag (the parliament) and the Royal Palace, you are very likely to see anglers standing in rows along the quays with fishing rods outstretched. Try your luck on your own or join a guided fishing tour"

Fishing in Stockholm during the day

Fishing in Stockholm during the night