I don't want and estimate, I want a quote and I expect them to honor it to the penny. Most boat work is fairly predictable for someone who is experienced and for the most part, I don't want someone inexperienced working on my boat.
I in come cases, I will ask how much it costs to give me an exact quote with work that has unknowns and then proceed accordingly.
Then there will be a work order, with a penalty clause for late performance and a bonus clause for early performance... with a lot of details, including the entire scope of work and materials performed. Especially, if they has possession of my boat.
Now, for known mechanics, which is the bulk of my hired work, I may bend, even with only a verbal agreement.
Has worked well for boats and planes over the years, with minimal surprises.