Oh, you'd have to ask the British rocket folks. Stewart I think his name was.
The rocket enthusiasts couldn't do regular rockets because of the explosives laws so they used water rockets. Then they were, IIRC, told that those were also considered explosive devices. I think it was sometime in the 1990's that the British Home Ministry was persuaded to change their interpretation.
But I might be all wet here. My memory never had time stamps and it's becoming more and more like a steel filing cabinet...full of mice. :-(
But I do remember that water rockets were for a time prohibited.
+McG+