Modelling realistic water is easy if you have the right glue...
This guide is about creating water itself. Make sure to check other guides on this website that use this technique for complete projects: river and sludge, puddle/mire/lake
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
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
Cover the water with a glossy varnish
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.