The only issue here is that, once they realised their quote wouldn't cover the job, they didn't contact you to provide a more accurate quote. What if they'd found that the only way to fit the machine was by extending the boat by 2 feet, would they have done that and charged you thousands, sounds ridiculous, but something a bit less extreme could easily happen if they don't communicate.
I took an old BMW to have new pads, when I picked it up the garage had done a full brake rebuild, new brake hoses throughout etc, and the bill was much more than new pads. I complained to Trading Standards but they said tough, I asked what if they'd decided it needed a new engine and fitted one, Trading Standards said that without a written detail of agreed works the garage could do whatever they wanted and bill me for it, perfectly legally..
Casp'