Even after a week in fresh water you are going have significant build up
of slime and algae that will not be easy to remove.

You must boat in some pretty sleazy water. You swim in that stuff? A week
in fresh water around here would not leave anything behind that would be
noticeable to any degree whatsoever. Three months would leave a mild
coating of algae that would come off easily with a pressure washer. Not
surprisingly, all fresh water is not created equal. I would refer the OP to
local knowledge at the specific marina he is considering to slip his boat.
The management at the marina or the mechanic's shop should be able to advise
him accurately.