I owned a Mainship Pilot 34 for 5 years and many other trawler and sailboats used in salt water over the years. These boats are designed to operate in salt water. There are some systems that deteriorate faster in salt water and others that foul with precipitated carbonates from the sea water.
Zincs in the engine and on the hull- transom, prop shaft and trim tabs keeps the under water metals in check. The front middle opening window which is mounted in a sheet aluminum frame deteriorates and blisters due to salt water. Scraping down to bare metal, priming with an etching primer and coating with an aluminum compatible finish coat keeps it under control.
Other deck metals are all stainless steel, except for maybe the windlass depending on the model which can have an aluminum casing so the treatment is the same as the windows.
Salt water used in cooling the engine, genset and air conditioning does foul heat exchanger surfaces faster than fresh. But a flush with Barnacle Buster or Rydlime takes care of it. You probably will also have to service the after cooler and it does deteriorate faster in salt water than fresh so read the Sbmar.com articles on aftercooler servicing and do it once. You will probably not need to do it again if you operate in fresh water.
So all of this work which may or may not have to be done will take a day or two of your or your mechanic's labor and then your boat will be fine for your fresh water use.