A good idea however with disappointing limitations is how I'm describing my latest idea.
There is no current way to measure the transparency of water, mainly because in the past there has been no need to, however it might come in useful in some ways to tell how pure water or any liquid is.
There could be a scale called VM (Visible Meters) and you would measure the VM of a water like so: There is a bright yellow pole going down into water (in this case, the sea) measuring the meters down from the surface of the water, the further down the pole you can see, then the more VM the water has. The North sea has a much lower VM than the Caribbean. So using VM you can tell how much clearer the water is.
If you found a new waterhole in the Amazon Rainforest and you wanted to know how clear the water is, they could add a chemical decreasing the VM by 50% so you only need something like a ruler to measure the VM and then double the results (or something like that anyway). Unfortunately, almost 100% of germs make no difference on how transparent water is so it would not help 3rd world countries identify what is safe water and what isn't.