That's why need to flush engine for at least 3-5 minutres after each outing specially if engine operates on salty waters, nevertheless although flushing should take care of salt deposits, it must be removed from time to time to have a flawless operation as you will always find adhered thin salt deposits on thermo due to constant on/off, cold/warm temp engine cycles.
When removing thermo for maintenance, do it gentle as not to damage thermo's upper rubber ring that can be damaged easily, soak thermo in pure vinegar for at least 15 minutes, gently scrape salt off with a stiff toothbrush around spring amd plunger as well, put back along a new gasket.
If heavily salted, crusted or corrroded put a new one in, engine will cool much better. Before doing so, remove all salt deposits on cylinder head thermo seat, apply water grease to seat & thermo's upper round rubber ring, that way will be much easier to pull out next time you want to perform a preventive maintenance.
Happy Boating
