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Fresh water is sure a lot easier on EVERYTHING including you than salt water!
A gel-coat "coating" will last longer and protect longer plus make washing and drying faster. In order to apply a gel-coat coating however you do have to have a clean base to start with and this means no previously applied waxes or sealants.
It's faster and easier to apply either a finishing wax or a light cleaning cleaner/wax by machine but it is true these won't last as long.
My write-up for this boat class shows applying a coating and the process....
Pictures & Comments: Marine 31 Boat Detailing Class - May 2015