What materials could be used instead of cement for a water-tight trench
Note that cement (or, rather, concrete) in and of itself isn't going to hold water. It's still have to be waterproofed. 

I don't know the dimensions or schematics, but my first thought would be a pond liner (basically, giant rubber sheets) and then topped with sand or other material to provide the traction.

I think a bigger issue might be to figure out how to maintain it, though. I imagine this water could get pretty nasty after a while without some form of filtration system.