I don't think it is likely to be "salt" (sodium chloride) that is causing the problem because:
- you wouldn't need vinegar to remove it - ordinary tap water would do.
- the sea isn't anywhere near saturated with salt - so it would disolved off just in sea water
- the difference in salinity between the belgian waters and the med is not huge - so this wouldn't explain the difference in observations (unless the belgian boat is in say an eastury).
- it would presumably have been worse on your recent trip to the red sea where salt content is higher
- mediteranean water is warmer than our water in the north of europe so it would be less likely to be a problem (more salt would disolve in the water / quiker / easier)
The need for vinegar suggests it is probably calcium carbonate (which is a salt, but not what we normally mean when we say salt). Calcium carbonate has an innverse solubility curve - unlike most materials which become more soluble as you heat the water up - calcium carbonate becomes less soluble in warmer/hotter water. calcium carbonate is what causes scaling in kettles, boilers etc.
It seems possible to me that your prop gets hot during use and so precipitates out calcium carbonate from the water.
Why this might be problem for you but not some other people could be related to different calcium carbonate concentrations in water, which can vary with geography/geology, co2 levels in the the atmosphere etc. and of course water temperature.