If the mirror is dry, corrosion is less of a problem. But mirrors in large scopes possess a big thermal inertia; they cool down during the night - then, in the morning, they stay much colder than the air for quite some time. That's the perfect recipe for dew to appear on the business end of the mirror. It's not just the salt in the air, but the air itself is probably more humid in your area - that means dew galore.
If you can afford keeping it indoors, then great, no worries.
But if you keep it in the garage, then dew will be a factor. Still, it can be avoided. The tried and true method that many large dob owners use is keeping a low-power incandescent light bulb under the mirror, to warm it up just a few degrees, enough to avoid dew. If the energy bill is a concern, use a cheap timer on the light bulb and turn it on only late-night to early-morning, when dew typically forms.
How to tell if the anti-dew method works? When you wake up early one morning, check the mirror out. If it's dry, it works. Just let it dew up a few times, it's not a huge issue, and then you could determine when to turn on / turn off the timer.
Of course, a mirror box design that is fully closed when not in use will help not only against dew, but also against mice nesting on the mirror and stuff like that; don't think it cannot happen. :) Even so, a gentle heating element under the slab of glass is a very good idea. Make sure whatever you do is fireproof.
There are also heating stripes available for specifically the purpose of dew-proofing scope elements. Just use google.
Also, even if you prevent dew from forming when in storage, you live in an area where the air is bad for the mirror. Dew that may form when observing will affect it. You'll probably have to clean the mirror more often than folks who live in dry areas. Let's say, twice a year.
You may want to use some silicon grease on the mirror cell and bolts, in places where two different metals are in contact (not on the whole thing - don't grease up the whole cell, it's pointless and disgusting). Hopefully, most of the metal parts are aluminum or stainless, and those metals don't care about corrosion (unless they are in contact with a different metal and a tiny galvanic element is formed at the point of contact).