Water is probably one of the most important things you can bring to a protest or demonstration. It keeps you hydrated, yes, but it can also be used to clean minor wounds, cool oneself and rinse chemical crowd-control irritants from one’s eyes. Virtually all water besides stagnant puddle water can be used this way, but water shot forth from a sport top-equipped water bottle is especially effective.
What is better at flushing things out of eyes and wounds—an aimless splash of from a “normal” water bottle or a robust stream forced from a sport top? The choice is clear, comrades. Of course, if you have an actual eye-flushing squirt bottle—like Beth recommends here—bring that, but a water bottle equipped with a sport top can perform double duty, hydrating and flushing in equal measures (if needed) without the need for any extra equipment. The directed, forceful stream means you can clean and flush more efficiently, leaving more for drinking later (or additional flushing and cleaning).
Just make sure your bottle has a top that you can remove and reattach so you can refill from a fountain or larger jug of water if needed. (Some of the flimsier ones can break, and a broken water bottle is a useless water bottle.)