Location: Los Angeles, CA / Settings: F6.3, ISO 100, 471 second exposure.
The most important advice I have to give about water and light-painting is that you have to get in it to get the shot.. Do what you need to do, but realize you will be getting wet.
For this kind of photo, being in Los Angeles, and all the reasons that our river water is not as clean as say Denver's right next to the Rocky Mountains, I would recommend a pair of these black rubber boots if you plan a similar shot.
For less polluted waters, you can probably go with a black pair of water-shoes instead.
Check out my ever-growing collection of Light-Skeletons on the image link below: