Its easy to damage the actual zipper, which often isn't corroded at all.
These require just cutting the old runner off, clean the area and clip on a new one. I dont know how long they last in a marine environment, but these people seem to have the answer - if they will last on a board bag.
Soaked the zipper fully in 5% vinegar, twice, and waited almost 3 days.
Still stuck and no visible changes in the "salt" corrosion A much sharper photo, but no difference in the zipper at all... The zipper to the left works fully, no issues at all - but it is also the zipper used normally and often.
I might be wrong but I am pretty sure that those slides are aluminum slides. So that is not salt or rust (rust is iron oxide only) but aluminum oxide corrosion. Soaking in a citric acid like lemon juice should work, but the ones pictured are so far gone I doubt anything would work. If vinegar did not work you probably will not be much more successful with citric acid, but if you are running experiments give it a try. I also believe they make a naval jelly for aluminum.
Peter is right. Some zippers are so corroded that there is almost nothing left of the runner. No coke or vinegar can fix that. My current Naish and North kite bags have no problems presumably because they are using a better quality zipper. My old F One and Flysurfer kitebag zippers completely disolved into nothing (these companies may have fixed this in more recent years).
If your zipper is only lightly corroded, try the solutions above and spray them with silicon to protect them. But ultimately it depends on the manufacturer to use salt resistant zipper materials.