Post #5 by
@sail sfbay on page 31/32 you get a diagram of the cooling system.
I would access the system at one of the points with a hose nipple that will allow me to push water through the system.
Just like I would attach a hose to my truck engine to flush the coolant system.
You’ll want to flush the water into a bucket or overboard.
I understand that the boat is new to you, but is all this DIY mechanical stuff also foreign. Some of what we say assumes you have a basic knowledge. If not let me know.
One of the best learning tools around is a basic engine class from a local community college. They provide insight into mechanical stuff that many of us here got in high school shop class.