You mention "water feature"... I assume you mean puddles? Obviously different types of water features look different and will need to be approached differently.
Here's a sample of how to do it for puddles/shallow streams...
http://daemons40k.blogspot.com.au/2013/09/swamp-bases-and-water-effects.html
That person used some Elmer's glue to initially create the smooth water finish, I don't really think that step is necessary, you just need a nice smooth surface over which you paint the water.
GW bases naturally have a texture to them, so if you sand them smooth where you want the water to be, that should be sufficient.
You just need to paint something that looks like water, have a look at google images for "puddles", "muddy puddles", etc to get an idea of what colours to use. Then paint that with a few layers of gloss varnish to give it a water-like shine.