A trigger_gravity set to 0, like cecil suggested, is fine. Be however aware, that while you can move at the x- and y- axis similar to moving in water, you can't change your speed at the axis in a 0-gravity trigger. So you can't controll your movements up and down, only sideways.
You could also try making a water-shader with an invisible texture, but then humans inside would look like they were swimming, and would drown of course after some time.