You need to use a Slow Field, which is crafted from one of the Workbenches (Tier 1 or 2, can't recall). You then float the slow field above the floor surface to the desired height (or depth, if you will) of the "water" using a floor safe or desk. Remove the item used to raise it, and voila, you have a pool. You need to place the Slow Field in a Defensive type room though, as it is a Defensive Item.
EDIT: Floor coloring and texture will help make the effect more "watery", but really any surface will work. The effect just may not be as noticeable over some surfaces/colorings.
Thank you very much, will try to get a room shot when I get it finished
Anyone able to tell me how to make a swimming pool with water effects? I've seen a couple that look very neat, but how do I do it in my supergroup base?