When the homeowner is at the point of hiring a painter, they generally will have colors selected or at the very least a color in mind. I always ask for the colors before I bid a job. Dark colors, high sheen colors and specialty finishes require more labor, this drives price. If its not a color change or I'm going over a similar color I give the pricing option of one or two coats. The best advise I can give based on 25 years in the business is to put it all in writing,colors, brands of paint preferred, when the work can be done, who moves furniture and how payment will be handled. I never get up front money. BTW you most definitely get what you pay for with paint. Higher quality products results in and better looking job. Don't be a cheapskate when it comes to paint or the painting contractor.
With that said, here's the reality of that particular scenario. Painters do put water in the paint, but not for reasons you would think. Some materials need to have their viscosity manipulated in order to slow drying time, allowing gravity to 'smooth' out the product for a better finish. It also prevents 'drags' and 'sausages'. I personally try not to do it too often, but from time to time I have to. I want my client to have a proper finish.
Maybe you've just moved into a new home, and you haven't got around to buying brushes and rollers yet. Perhaps you're worried that your brush-skills aren't that good, and you won't achieve that professional-looking finish that you’re really going for. Handy professionals will turn up with everything they need to get the job done, from ladders and brushes, to rollers and tarps. You just have to provide the interior paint and primer!