Documentation of your HighWaterLine is very important! It is an ephemeral and performative project that doesn’t last very long in the rush of life. Having good images that you can share after the fact can be invaluable for your own records but also for media that may come to the story late. (The majority of the coverage we have gotten has been after the fact).
Create a shortlist of the photo and video shots you want to capture as the artwork is being created. Some examples are iconic locations, the marking of the line, and busy neighborhoods. Consult with the people doing the documentation so that they understand what you are looking for, and the kind of shots you want to capture.
You will also want to make sure that you have photo releases signed from all of your participants, and either get on camera (video) or signed releases from people that you document interacting with your team. This way you can have full confidence in sharing your images with the media.
Ask your photographers to license their images under a Creative Commons license that allows reproduction of the images with proper attribution, such as a Creative Commons Attribution Share Alike license.