How to catch the impression of that really pure and clear water.
How do you best shoot these images, which let's the freshness of the water come out in the final picture?
Well, first of all you find clear water!
Yes, I know it sounds obvious, but if the water is muddy or silty, it will probably never come out as clear. Some water has a tone to it - blueish, greenish or peaty, tea-colored, but is still clear. The color is mostly caused by different substances diluted in the water like calcium, ocher, humus from bogs or from particles like a green tint caused by algae or brownish caused by small particles. Such water can still appear clear, although it's not going to come out stunningly clear.
We usually perceive the "fresh" colors with clarity, so the blues and greens will work better than the browns. Many of my own clear water images have been shot in peaty waters - mostly because we have a lot of them in Northern Europe and Scandinavia.
But for the breathtaking clear look, you need clear water. Oceanic, spring fed or similar. Clear as gin.
So, when the water is found, you have two things to see to for bringing that clarity into the picture: light and reflections.
The light generally needs to be in you back. This will give the best light on the stuff in the water - typically the bottom consisting of sand, stones, tree trunks or whatever. If the sun is shining, just watch out for your own shadow.
You can also move up a bit - or simply lift the camera. When your angle of view is high, you can sometimes avoid the refections too. At least in part of your image. A camera with a wideangle lens stretched out above your head can give some amazing impressions of the clarity in the water, while at the same time retaining a great perspective with foreground and background in your image.
You can also use wizardry such as a polarizer. That is a filter, which has the purpose of removing reflections. By putting that in front of your lens and turning it to the right angle, you can filter out a lot of reflections.
The flash will illuminate stuff between the camera and your subject, and the camera will have a tendency to expose for the nearest elements, and not show the stuff further away, completely ruining the sense of perspective and translucency in the water.
So underwater images are best shot in really clear water with lots of light. And since you usually can't see what you are doing, you'd better take a lot of them, because most are usually useless.
When over the water, the flash can be the opposite: a great helper to penetrate the water and get to see what's down there. Flash images taken at dawn and dusk have a tendency to bring out a really great ambiance in clear water, and you can sometimes flash almost straight into the water and get some great results. But don't go all straight. That will only result in reflections of your flash. The same rule applies to angle as above. A higher angle gives a view into the water, a lower angle will let the water appear all dark.