Modelling realistic water is easy if you have the right glue...
Note that we are creating the water surface not its bed so it must be almost perfectly flat
1. Even the water surface by pouring a thick layer of polymer/nitrocellulose glue easily available from DIY stores. You may theoretically just use a PVA glue but then the base of your model may warp ruining the effect
2. Paint the water with a grey paint with a hint of green and blue. Darker in the middle with brighter shallow areas. I use Games Workshop's Shadow Grey
Note that real water is not blue or green. Physically it is black as it absorbs almost all light. Artistically you may call it dark cyan. Various pollutions may alter its colour but it is still mainly dark grey.
3. Cover the water with a glossy varnish
4. Use drybrush to paint some yellowish-green duckweed at the edges
I don't use ready-made modelling water as it is expensive and actually worse then a glue covered with varnish.