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. (You never use a sprayer and need to thin paint.?) If i was still in the industry I'd take the time to make a better article than this. Take this for a grain of slat. Use a reputable painter or someone you know you can trust or has been referred to you by someone you trust. I wouldn't hire anyone I had to watch like a hawk to make sure they're not screwing me.
This is an area that interior painters can easily cut short to save time. Most contracts don't state the extent of the wall repairs included, so it's up to the painter's discretion how much they will do. If it seems like too much work, they'll usually point out the repairs but don't include it in the bid. They'll then ask you if you'd like it fixed after they start your project and let you know how much more it will cost.
Some proposals simply say to paint the walls and ceiling and never specify the number of coats to be applied. If the colors are similar enough, it's possible to get away with one coat of paint and not discount your pricing. No matter how hard you try, tiny, pin-sized air holes will pop exposing the original walls. This may not bother you if you can't notice it, but principally speaking you should have paid your painter less for the work.
Thanks, all, for your time & efforts adding to the article & comments, especially Dave urging requesting both General Liability AND Worker Compensation insurance certificates to protect from real & fraudulent liability--from my experience especially in California, where insurance fraud is a popular income thief, even causing car collisions to collect.