It is not a very big deal to change a "sea water " cooled to "fresh water" You need a heat exchanger
and a water pump and a few hoses and clamps and other bits. Since often if it near impossible to find a way to drive a second water pump off the engine, the second water pump is typically an eleectric one. I find that bait pumps work well. I typically use the bait pump to pump the cool sea water. I then use the original sea water pump to circulate the hot "fresh water conecting it so the suction of the that is connected to the output of the heat exchanger and its output is connected to the original input to the engine. A relay and fuse in the electric
supply line generally is needed. Lots of luck.