When you say "fresh water
cooled" I think you mean antifreeze
in the closed cooling
loop, and a heat exchanger
system for cooling
Which means two parts
to flush. Our engine
manufacturer recommends their own product for flushing
the closed coolant
system. Happens to be Cummins Restore, and our service
manual has directions (mostly drain old coolant, fill with Restore, run it for a bit, drain the Restore, fill with new coolant).
The second part is the raw water
passageways, heat exchanger
, etc. Two products marketed for this purpose are Rydlyme and Barnacle Buster, and there may be others. One method is to fill/wait/drain, and another is to recirculate, latter said to be more effective.
In your case, probably useful to learn about any restrictions Kubota may publish...
I used Rydlyme on our engines, one each method, and on our genset, 2nd method, didn't see much difference. I've used Barnacle Buster on our ACs, first method, seemed to work
. Recirculation in this case would be a major pain, 'cause catching the outflow isn't easy when our boat
is in the water.