Types of Survey
Pre-purchase surveys are carried out for the purchaser when purchasing a yacht or commercial vessel and can be carried out in the water as was the case with the SD Melton pictured below, which I recently surveyed for a Christian charity that will be converting her to a medical ship for use in Madagascar.
A full survey includes the underwater sections, so requires part in and part out of the water, ideally with a sea trial also, to be able to thoroughly access the vessel's structure, build quality, and all equipment, as with the Schooner Encounter pictured below during a survey in Greece.
A valuation survey is an assessment of a vessel and her equipment, in an attempt to determine her current market value based on similar or comparable vessels on the market, using industry tools such as soldboats.com and experience.
Required by the vessels insurance company at various intervals depending on the age of the vessel, her rig/equipment and intended use.