For this water effect, there's no escaping the Pen Tool!
Since your dealing with two different colors you will need two different paths. You can set the tool to shape or path. If you use shape you need to pick the color of the shape (which can be easily changed later) and lower the opacity so you can see your work. You get much cleaner results with the shape option.
Here I have used the path option just to quickly demonstrate.
Using the Pen Tool set to path, on a new layer, I created the first water color/shape.
Right click and make the path into a selection.
Then fill the selection with the appropriate color.
On a new layer, repeat the process for the second water color/shape.
Pen Tool path....
..turned into a selection...
......then filled with color.