• One room or the whole house? Applying one coat in one room is a reasonable DIY Saturday project (especially if you have help and beer). Multiply the time spent moving furniture, prepping walls and sanding old trim by the number of rooms in the house, and you might want to hire real help. It's the same outside. You can probably tackle one shady garage wall that needs a little scraping and sanding plus a coat of paint, but covering all surfaces of the house is usually best left to a pro.
Contractors apply several layers of paint to achieve a suitable finish. They leave the first coat to dry for the recommended time and apply one or more finishing coats. They might choose paints with special qualities for different types of room. Manufacturers have developed paints for bathrooms or kitchens that have good resistance to moisture for example. When they have completed painting, they clear away any equipment and restore the area to its original condition.
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.