There's a further reason why drinking lots of water helps. Part of the 'flushing out' effect is because, if you don't get enough water, your kidneys can't do their work properly. As a lot of their job is filtering the liquids you ingest, non-water beverages can actually mean your kidneys have to work harder.
If your kidneys can't keep up with their workload, the liver has to help out. Which means that the liver can't do it's own jobs as effectively. And as the liver's jobs include processing vitamins, proteins, fats and other nutrients, regulating some hormones and enzymes, and processing any alcohol and other toxins you ingest, if the liver can't do it's job effectively, your metabolism slows doen and you're not going to lose as much weight.