Not only careful communication is essential, but we would greatly recommend, when you pick out your type and brand of paint that you go and pay for it, with the contractor's discount, which is given by Sherwin-Williams and most paint companies. Then you will know how much paint you purchased and the coverage. If it is possible, inspect the work of the painter daily.
Keep an eye on the new cans as they're being brought in. Make sure they look new and don't have paint in the rim of the can. If it's a five-gallon bucket, check to see whether the lid is still sealed on with the plastic strip. The only time it's acceptable to mix water in the paint is when you're using a deep or ultra deep base paint to reduce its stickiness, which is rare with new paint technology. Dark primary colors are composed almost entirely of tint that makes it very hard to work with without adding water.