Any metal part of a zipper will corrode with enough exposure to salt water and air.

Frequently wash any items that come in contact with salt water.

Some slightly seized zippers can be freed by rubbing off the corrosion with Coca-Cola and/or rubbing alcohol.

If the zipper teeth and tape are in good condition, remove and replace the slider.

If the zipper teeth and tape are damaged, pursue a full zipper line replacement.