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Electrolytic damage
When surveying GRP vessels the process will always include hammer testing. This is undertaken to establish the external hull condition above and below the waterline. 
This hammer testing causes no damage whatsoever but the experienced ear of the surveyor can identify various characteristics of the structure.
Distortion of the structure at any point, misalignment of keel/rudder,
Hull laminate and gel coat cracks and their cause.
Evidence of keel movement and internal damage.
Hull moisture meter readings. Deck moisture meter reading if relevant (cored decks).Deck distortion caused by mast and rigging tensions.. Internal bulkhead condition. 
Hull skin internal.
This is just a fraction of the areas that have to be inspected. See sample surveys HERE for full list
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electrolytic damage
Timber construction 
Timber construction 2
Cockpit and masts 
Moisture meters and osmosis
Cored decks
Gel coat damage
Rudders and masts 
Timber construction can be very complex and the issues that can occur need to be fully understood by the surveyor.
Hammer testing is undertaken throughout the structure both above and below the waterline and both internally and externally including the deck and superstructure. 
This may also be accompanied by spike testing if the hammer testing highlights a problem. Spike testing should not be very noticeable on sound timber and would only be used to confirm the hammer test concerns. This will locate freshwater decay and one or
two other common problems.Decayed timber will be found if accessible and suspicious timber reported.
Fastenings
Timber hulls have a considerable number of fastenings, and these need to be described. All of the important fastenings are completely hidden but some of these do have to be replaced or examined occasionally, 
On survey, one or two of these will be removed below the waterline with owner permission. These are known as the hood end fastenings. Usually bronze screws holding the ends of the planks to the centreline or transom structure. It is surprising how corroded these often are, yet they are holding the planks on.
Rot and decay would appear to be the most concerning of these issues but surprisingly enough, the biggest issue with timber vessel hulls develops below the waterline and not because of "rot".
Electrolytic decay is the single most common and expensive fault usually found and the single easiest problem that can be started by misinformation. Electrolytic decay occurs when
two differing metals touch in the presence of seawater An electric cell is produced and an electrolyte forms around the metals, This electrolyte destroys the timber that surrounds the metals that are touching. The most common cause of this is anodes that are planted on the hull skin and wired to seacocks, stern gear and any other "protected" metal. When the anode is wired to the internal stern gland as most anodes are then the timber surrounding the stern tube internally simply turns to fibrous mulch.. The first sign of this is always a gradual build up of white soft crud around the anode fastening internally and around the stern gland internally. This means that damage to the timber is already occurring.
Broken Frames
All timber boats will have frames of some sort, Mostly these are steam bent frames of oak or Rock Elm. Most timber boats will have one ore two fractured frames, sometimes these will have been present since original build and in isolation are not too much of a concern.
Below fractured frame on Clinker built boat. 
The surveyor should try to locate any broken frames and advise on their importance./ Frame fractures on adjacent frames and on the same plank seam would be considered important and in many cases, repaired.
Steel hulls have their own special needs. Rust is the obvious enemy. A steel hull is built with a considerable amount of internal support structure, usually welded to the plating. Steel hulls internally suffer with condensation and this condensation collects around these hull supports, angle iron welded edge on to the plating thus forming water pooling areas.
Because of this condensation most steel hulls are lined to insulate the hull, however this does not always protect the plating from getting wet beneath the insulation. This is where the plating rusts.
Externally the hull is hammer tested at very closely spaced intervals, one strike in every 2 square inch box throughout the hull . Even this can miss thin plate. The hull is also ultrasound tested similarly close spacing but a steel hull can literally have 4mm thickness less than an inch away from an area of 1.5mm thickness due to corrosion. Hence one can see why steel surveys can be so time consuming.. Thick paint coatings can inhibit ultrasound testing so a special through paint tester is used where required.
The hull is examined for excess distortion, few steel hulls are perfectly fair.
Bronze skin fittings can be a major problem on steel hulls
due to mixing metals and bronze/steel connections are very vulnerable to problems. It will always be the steel that suffers.
Few insurers will accept a Ferro hull other than 3rd party but some professionally built hulls can get full comprehensive.
An exceptionally well built ferro yacht
Basically Ferro hulls can be enormously strong and a survey would use hammer testing on the hull skin externally to ensure there is no crushed cement, no fracturing and no acid attacked concrete. The survey also checks for rust bleeding from the steel armature within the hull shell. A well built ferro hull will be as fair as a grp hull, if not, that might suggest a non professional build with the other possible implications of poor quality in the shell that are impossible to discover until failure occurs, which is why insurers are reluctant to insure Ferrro hulls. Nevertheless the process of inspection follows the best method to check the integrity of the hull and standard of build.
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This page describes the process of starting the survey process and what to expect from insurance or pre-purchase surveys.
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