Microdol X supposedly used NaCl as silver solvent, so obviously sea water does something to silver halides. It is well known that silver chloride forms soluble complexes with extra chloride ions, and a huge load of chloride ions like from the solution suggested on that caffenol blog could do just that.
There are a few issues, though:
- Most modern film emulsions don't use silver chloride, but a mix of silver bromide and iodide. Fixing of these is much trickier than fixing silver chloride because silver bromide/iodide is much less soluble.
- Making concentrated NaCl fixer usually means table salt + tap water, and table salt usually contains some iodide. I have seen 1g/l iodide (forms soluble AgI2- complexes) in some ultra rapid fixer recipes, but optimal amount may be tricky. Using the wrong amount of iodide might convert somewhat soluble AgCl into completely insoluble AgI.
- Saying it cleared film doesn't automatically imply archival fixing. Even if the film looks great today, who knows what it will look like a year from now? If NaCl is the only option at a given moment, I'd still keep the film somewhere dark and I would definitely refix it after return to civilization.