Here's a tutorial for how to make a small marsh pond.
Take an old CD. Cut out a bit from thin plastic (plastic from a blister pack is perfect) to cover the hole in the middle. Glue that on the underside with super glue.
Take some sculpting putty to fill the hole, and take care to fill the groove around it as well.
Sculpt some ground around the edge.
It should now look something like this:
Use PVA/wood glue to glue sand to the ground.
Prime and paint with marshy colours. I prefer to paint the water green and brown and not blue. Varnish it all, matt on the ground and gloss on the water.
Add static grass and/or ground scatter to the land.
The end result should look something like this: