Poser doesn't do reflections, so the only way to accomplish this effect is through trickery or postwork.

If the scene isn't too complex, you could create a duplicate, flip it vertically, and place it under the floor/water. If you make the surface partially transparent, it'll look like a reflection. This sort of trick is sometimes used in computer games.

Another approach is to make the floor invisible, then position the camera beneath the floor, and render the reflection separately. You'll have to use a paint program to flip the reflection. Then you can render your image and somehow composite the reflection into the scene. Or you can make the floor transparent and import the reflection as a background image.

The latter technique (rendering the reflections separately, and importing them as backgrounds) is how I created my reflection animations in Poser, although I was importing videos rather than still images.

Look here for an example of this technique.