Actually, no surveys are needed to buy a boat. However, having a survey is highly recommended, unless you know the boat really well. Depending on how large the boat is, and how complicated the systems are, and how old the boat is, a separate
rigging survey and engine survey may also be wise.
Most surveyors don't dive... in fact, I've never heard of one doing so.
The will generally do the survey in two-parts. The first part is with the boat out of the water, and includes an inspection of the hull's exterior, prop, prop shaft, cutless bearing, the interior systems and parts of the boat and possibly a visual inspection of the
rigging at deck level and sails. Then the boat is usually launched and the sea trial portion of the survey commences. This includes putting the engine and transmission through their paces. The is especially the case if the boat has been on the hard, as it is easier and more accurate to take moisture readings from the hull if the hull isn't wet.
Of course, some antifouling
paints interfere with the moisture meter's ability to read properly.
A separate
rigging survey, done by a rigger, might include dye penetration testing of the swages, going aloft to inspect the masthead fittings and sheaves, the spreaders and spreader bases, checking the tension on the
rigging, etc.
A separate engine survey, done by a diesel or engine mechanic, should include visual inspection of the cooling and
exhaust systems, a compression test of the engine cylinders, etc..