Radleigh, There are many different ways to make water for dioramas, but for large and deep water, 2 part (resin and catalyst) polyester resin is probably the best choice. This stuff can be found in art supply stores, auto part and boat shops, craft stores and hardware stores. It is normally used to cast objects from molds or to make fiberglass objects. It can be tinted with available dyes or even food coloring. Follow the directions carefully. Too much catalyst and it will crack and craze. Too little and it will remain sticky forever! Work in layers of only about 3mm at a time. Paint the basin in which you want the water a muddy brown, lightening towards the edges, and let thoroughly dry. Mix and pour, then pick up the diorama and gently tilt it to let the resin flow into all nooks and crannies. Set on a level surface to harden, about 48 hours.