James,
I doubt whether hiding the car in a garage will alter the fact that you're living in a marine environment.
Some technical stuff to wade through...

http://www.galvanizing.org.uk/?cms_id=3 ... 0Corrosion

Within a given local environment corrosion rates can vary markedly. For example,
steel may corrode more on a particular side of a building because it is in the shade and
so remains wet for longer periods. Prevailing winds may carry airborne contaminants
(e.g. exhaust from a power station or sea spray from the coast) predominantly on to
one face of a structure. It is therefore the “microclimate” immediately surrounding
the structure which determines corrosion rates for practical purposes. Steel is most
commonly used in the following environments:
• rural atmospheric – essentially inland, unpolluted environments;
• industrial atmospheric – inland polluted environments;
• marine atmospheric – the 2 kilometre strip around the coast is broadly considered
as being a marine environment;
• marine/industrial atmospheric – polluted coastal environments;
• seawater immersion:
(i) the splash zone, immediately above high-tide level, is usually the most
corrosive zone;
(ii) the tidal zone, between high tide and low tide, often covered with marine
growth; and
(iii) the low-water zone, a narrow band just below the low-water level;
• the permanent immersion zone, from low-water level down to the sea bed;

Doh

Luckily it's not a wild sea but a harbour

I think that it will help. Sometimes when the wind comes from the sea my car is covered in a layer of salt.