[tempest]

Primarily, As Mike says, the difference is simply the corrosive nature of salt water and chloride laden air in the environment that attacks metals through the electro-chemical process. The water itself becomes an electrolyte and it's also corrosive to engine parts, welds and crevices etc. It's a constant battle to inhibit this process, using anodes, coatings, materials etc. Anaerobic corrosion can occur where stainless steel, for instance, gets wet in places starved for oxygen in Chainplates, Wire rigging etc.
Marine growth also helps create an environment for degradation.

How well an owner fared against this constant battle should show up on close inspection.