What salt actually does to an outdoor coil
A condenser coil is aluminum fin bonded to copper tubing, and those two metals sitting together in salt laden humid air corrode faster than either would alone. The damage starts at the joint between fin and tube, exactly where you cannot see it, and it builds through seasons of ordinary operation. Corrosion products pack the space between fins and restrict airflow, and pitting in the copper eventually opens pinholes.
That is why the first symptom is almost never a dramatic failure. It is a system that seems to lose a little charge every summer. By the time the leak is large enough to find quickly, the coil has usually been corroding for years.
Rinsing, coatings and where the unit sits
The best defense costs nothing but time. With the power off at the disconnect, rinse the outdoor coil down with fresh water on a regular schedule through the year. You are washing salt off before it has months to sit there. Do it more often if your house is close to the water.
Two other things matter. First, at replacement time ask about factory coated coils, which are made for coastal installations. Second, look at what is hitting your unit. An irrigation head that sprays the coil every morning is putting mineral deposits directly onto it every single day, which is worse for the coil than the bay air is.
Bay adjacent houses and where equipment lives
Close to the water, elevation is part of the design conversation. Outdoor equipment set on a low pad in a surge exposed area is going to get submerged eventually, and a submerged condenser is generally a total loss rather than a repair. Raising the unit on a stand is worth discussing at replacement time.
The same thinking applies indoors. If the air handler and its electrical are at floor level in a home that has taken water before, ask whether the replacement can go higher. It is a design decision that is nearly free at install and expensive to change later.