If you can’t find suitable images, or they are too expensive, then working with graphics could be a great option. In this example I turned an image of the water surface into a graphic header.
Working with blurred images
You could also work with blurred images with some ‘bokeh’ that effectuate a certain atmosphere.
Night photography works especially well here, you could also enlarge a portion of the image and use it in other places. The important part is to have an image with some depth to it. This conveys a more open than closed space which is preferable over making your visitor look at a flat surface.