From the wiki article:
"As water rockets are capable of breaking bones upon impact, they should never be fired at people or animals."
I know you said indirect fire but you gotta count in accidents
"Water rockets should only be launched in large open areas, away from structures or other people, in order to prevent damage to property and people."
Ours can in theory be used anywhere.
"A typical two-liter soda bottle can generally reach the pressure of 100 PSI safely, but preparations are made for the eventuality that the bottle ruptures."
No thank you, I'd rather not that explode in my face.
"While pressurizing and launching the rocket, bystanders are kept at a safe distance. Typically, mechanisms for releasing the rocket at a distance (with a piece of string, for example) are used. This ensures that if the rocket veers off in an unexpected direction, it is less likely to hit the operator or bystanders."
Again, ours can in theory be used in proximity to people without harm. A 12 gram Co2 cartridge exploding is bad, but it would be contained by the barrel for the most part. A 2 liter water rocket explodes...
On the other hand... that Aerial Photgraphy idea might come in handy as a recon device, especially if you can somehow get a remote feed off the camera.