The lowest salinity levels of the St. Johns River are between Green Cove Springs and Palatka, recently recorded by NOAA at 1.8 PSU. A ‘Practical Salinity Unit’ is approximately 1 gram per liter of water. So that’s pretty low. Up around downtown Jax, the salinity level can be triple that, but this will vary according to the tides. Salinty rises pretty steadily as you approach Mayport, as you might imagine. If you spend most of your day on the river around here, does that mean you don’t have to rinse out your raw-water cooled engine after a day on the water? It’s always a good idea to do that, if not to remove any salt, but other particulate matter that may have accumulated.