Even colder than it looks
As any good high-school chemistry student knows, you don't describe waters as "clear" because it's technically "colourless". Here in Norway not only can the water be described as "hand-numbingly cold" but as as extremely colourless. It's amazing how crystal clear water is when it's fresh off a mountain glacier.
And even in towns, where rivers tend to be a brownish (aka Thames brown) colour the water-ways here are still completely transparent in a rather creepy way. As you cross bridges you can count the many bicycle carcasses on the river bed below.