"The smartest man in the room is the one who admits his own ignorance."
I've owned boats, airplanes, RVs... have a couple of degrees with the word "Engineering" on them, and even do a lot of my own maintenance on them. But when I upgraded from trailerable-size boats to a cabin cruiser I decided to hire some expert advice. I think there's a world of difference between a DD ski boat stored on a trailer, and an outdrive-driven cabin cruiser that lives (permanently) in the water.
For me it was a chance to quiz the surveyor for advice, and his presence set a minimum standard that the boat must meet before the purchase went thru. The owner was quite honest and likable, but made several repairs unasked when informed of the impending survey (bellows, shift seals, steering gimbal, and gimbal bearing). It took almost an entire day, starting with the detailed exam of the hull (in the yard) and concluding with a thorough sea trial (idle, shifting, planing, WOT rpm, and various measurements of the engine while at cruise.. surveyor spent half the time head-down in the bilge). The surveyor was also VERY familiar with the local repair yards and was able to evaluate most of the maintenance records based on accumulated local knowledge. He was able to assure me the boat had had good maintenance throughout its life, and had been regularly used*. At the conclusion, he presented his findings and quietly said: "If you don't buy it, I might. It's in better shape than 99% of what I usually see."
*I was looking at boats 10-15 years old. He strongly advised against low-time boats in that age. Too little use was worse than too much, according to him.