In salt water service corrosion is your enemy with RWC, especially if the sacrificial zincs have not been religiously maintained over the years. Thinning of the water jacket walls can lead to leaks within the cylinders and heads.
RWC engines can be converted, but it's a bit costly, just starting with a quality pump (Jabsco etc) can run $300 or more. Heat exchangers may be found from a variety of sources. Add to that brackets, pulleys, some sort of reservoir and I expect the cost will add up. And do you have the space to add all that equipment?
The other issue is if its an older engine, and is in questionable condition due to long service, converting now may be an issue of being a bit too late. However, theoretically you will put a halt to further weakening.
It's ultimately do-able, but is it really worth it... that's the question.