I was definitely going to use the 33 meter but my concern is that it seems that the boat would need to be completly dry before you could get any accurate measurements of the Hull. No dew or any other moisture of any kind on the outside or inside. I don't know how its done in other area's of the country but around here a survey is typically done in the slings and the boat is not out of the water that long. Maybe it really is like VtSailguy says, you just roll the dice and hope you don't roll snake eyes.
(now that I am back on a PC, instead of that damn phone I can reply...)
Not that you have not already done so, but I recommend that you take your CT-33 meter and thoroughly familiarize yourself with its behavior while checking through bottom paint. Use it on the bottom of a bunch of boats in any yard. Scanning won't hurt the boats, but you will get a feel for when you can, and cannot use the meter.
Many (most?) bottom paints contain copper. Copper, or any metal, will give you a high reading whether or not moisture is present. (The calibration plate for the meter includes a copper film to give it a known reading [14%] at a specific distance from the back of the meter.) The meter may also pick up metal (backing plates, chain, pie plates, fishing gear) that is stowed inside the hull. That being said, you should still use the meter to look for variations in the moisture readings, and then look to rationalize the variation.
In the pic above, I simply scanned along the outside of the hull while walking next to the boat. I noticed the meter as it jumped, and then scanned around that area more thoroughly. Those circles were about 6" in diameter, and they were very subtle. If not for the meter, I don't think that I would have discovered them.
YES the boat should be out of the water for at least a day - longer is better. Since your location and cruising area are secret, I don't know if boats in your area are hauled for the season. However, if I were considering taking ownership of any boat with a cored hull, I would wait to inspect it until after it had been pulled for the season, or spend the extra $200-300 to have it hauled for a couple of days. I would not want to end up paying for a $10K-$100K hull fix.