I know a "little" about this, so keep that in mind.
It's a handheld ultrasound machine with a probe which the operator uses to read the minimum hull thickness. The "standard" is looking for less than a 20% loss in original plate thickness. Audiogauge/ultrasounds can be performed with the boat in the water and it reads through the paint and minimal corrosion, but the surveyor picks out the most likely spots for corrosion, such as under the shaft drip seals and any tanks or obvious scaly rusty spots. The (most) correct way would be to check as many plates as possible, and sometimes these are unaccessible while the boat is in the water. With the boat hauled out, obviously the surveyor could check the entire hull if needed.
If you're the buyer, and you hire a surveyor, expect to pay. The report SHOULD NOT be given to the boat owner, for him to peddle to the next prospective buyer. Afterall, YOU paid for it. The surveyor is working for you. (Obviously, if the boat owner comes up with a survey, that surveyor might not have the buyer's best interest in mind, nor might it be current enough) Likewise, the insurance company and the bank/financier will most likely require a copy of the survey before writing a policy or cutting a check.
Maybe a marine surveyor can chime in here to add more insight into the whole process.
