i have a 1gm10yanmarraw water cooled and i asked the same question to yanmar. they told me that these engines were designed to run in a raw waterenvironment and that it would be a waste of my money to convert it over to a fresh water cooled system. this coming direct from yanmar bears a little credibility i think.
Absolutely, hence the them using the word seawater and not freash water to describe your cooling system. Cooling systems with heat exchangers are often called fresh water cooled. Make sure you have the proper zincs installed on your engine to avoid electrolysis. Your Yanmar manual should tell you where those are.
They have a lower temperature thermostat. They are made for the environment. You could easily install a flushing T to flush the salt water out between trips. As unbusted stated: proper maintenance is key. Do you have a manual.
You can find links to one on the site.
Make sure you work the engine, don't baby it, maintain it well and it won't let you down. It should have max RPM at 3600 and cruising at 2600-3000.