How much faster does an AC really fail near the Gulf?

It varies by exact distance from the water and how exposed the unit is, but it's common to see meaningful corrosion damage in half the time compared to an inland system — sometimes less.

Can a corroded coil be repaired, or does it need replacing?

Minor surface corrosion can sometimes be treated and coated. Once fins have deteriorated enough to restrict airflow, or copper tubing has pitted through, repair usually isn't cost-effective compared to replacement.

Does salt air corrosion void my manufacturer warranty?

Many manufacturers exclude corrosion damage in coastal zones unless the unit has factory-installed coastal protection. It's worth checking your specific warranty terms, since this catches a lot of coastal homeowners off guard.

Is there a type of AC unit that resists salt air better?

Yes. Units with factory-applied coastal coil coatings and coastal-rated cabinets hold up noticeably longer than standard residential models. If you're due for a replacement, this is worth asking about upfront, before the first corroded coil.