By Chris Caswell.  http://www.yachtworld.com/boat-content/2010/11/survey-says-hire-a-surveyor/
Most of us would be able to spot obvious structural problems with a boat, but many issues are too subtle for anyone but a professional surveyor to catch.

It was an eye-opening experience because, as a long-time boat owner, I thought I knew a lot about boats. I was buying a used 25-foot fibreglass sportfisherman, and I’d already spent most of a day crawling around the bilges and poking into the corners, and I had given the boat a clean bill of health. But my insurance company had insisted on a marine survey, so I was following the surveyor as he examined the boat. A tiny crack in the gel coat at one corner of the cockpit led him, just like Sherlock Holmes in pursuit of a clue, to the main cabin bulkhead. I’d examined that same hull/bulkhead joint too, but the surveyor found another hairline crack that indicated the whole bulkhead had broken loose. In fact, all the bulkheads had “let go” from the hull to some extent, and they were so hard to spot that I’d missed them completely. In a few minutes, that surveyor had saved me more than 40 times his bill!

Not all serious problems are this obvious, which is why a surveyor is a must-have for used boat buying.