This is how I went about creating it.
The base of the lake is plywood with shaped sides made from Styrofoam and drywall plaster. I painted the center of the lake black to make it look deep and used brown around the edges. At one end I glued down some fine sand to make the beach area.
The water flows out of the lake via a weir and I ensured that I left a channel so that the weir kit could be inserted. I installed the weir and plastered the area around it to match the sides of the lake.
I planned to have a small rowboat in the lake but did not want it perching on top of the water, I wanted it to be in the water, so I decided to glue it into place before I added the water. I glued the captain, his oars, and his passenger into the boat and glued the boat to a small length of plastic tube to hold it the right height above the base of the lake.
I also placed swimmers in the shallow area at the beach.
For the water, I used EnviroTex Lite - an epoxy resin that sets hard and clear. It has to be applied in layers not more than 1/8" thick so it took a number of days and pours to fill the lake. Each time I carefully vacuumed the lake to ensure there were no bits of dust.
Once I was almost at the desired level, I added a pair of kayakers so that they would be sitting in the last layer of water. As can be seen I also started adding vegetation around the lake.
I made a safety barrier out of some Zulu beads, to prevent swimmers from going into the weir. I laid it across the mouth of the weir in a curve that matches the water flow going towards the weir.
Unfortunately, even though I vacuumed thoroughly before pouring on the last layer, I had used some static grass nearby and I did not realize that it clung onto the resin despite the vacuuming. The result was a layer that had lots of nasty bumps and spots on the surface. I could not add another whole layer as it would have swamped the rowboat, so I tried to 'paint' on a very thin coat of the resin. This was only partly successful and a few blemishes remained, and I also missed a few spots, so I had to repeat that step.
I added the stream that runs into the lake, some reeds and trees, and of course the rest of the beach goers.
A small ice-cream stand provides drinks and eats as well as handling the boat rentals.