When looking for a body of water to reflect you need to have a very still surface. It needs to be like glass or a mirror and the water needs to be as dark as possible to give the maximum amount of reflection.
There are commercially available dyes which allow you to change the colour of the water. This was very popular at last years Chelsea. Sometimes you do not need to dye the water but instead can use a very dark coloured substrate instead. Using just a black liner, with its inherent folds and pleats, is not the most aesthetically pleasing way to go.
Reflective pools are great for giving a new perspective on adjacent structures and buildings. They can also be used to create a bold statement with striking planting around the pool that is mirrored in the dark water.
The water quality needs to be pretty good, especially if you have fish in the water as they will tend to break up the calm surface from time to time. Sometimes you do not want the water to be like a mirror. Movement can again give some interesting effects, although this is not the traditional perfectly smooth view. Fish can also give rise to waste material and even bubbles in the water that detract from the simple smooth effect that you are trying to achieve. Similarly, waterfalls and fountains are best avoided as these will again create surface disruption and bubbles on the surface.
Most of the time a still surface is the key thing with dark water that reflects well. Creating this whilst incorporating fish or other features can be difficult and is always a compromise so think carefully about what you aim to achieve from your reflective pool.