First remove the petcocks, also known as engine block drain plugs. They are on the side of the engine opposite the spark plugs, in the area circled in yellow below.
Below is a picture showing the petcock plugs prior to their removal.
When you remove the petcocks/drain plugs, water should begin pouring out into the bilge. This is the desired result. Below is a picture of the petcock plugs after they're removed.
And below is a picture of the engine block with the petcock plugs pulled.
Very important: once the petcock plugs are removed, take a small screwdriver or piece of wire and insert it into the drain holes. This will ensure the holes are not plugged with residue from the raw water. Occasionally the residue will be so severe, no water will come out when you pull the petcock plugs. Use the screwdriver or wire to remedy this issue.
Allow the water to drain for as long as necessary, it should take a couple of minutes. Then rock the boat back and forth for a minute or so, which will allow more of the raw water to drain.
My 3.0 manual refers to four petcocks, but this engine only has two. I am not sure why the manual is wrong about this, possibly because it is the same manual for the 5.7 liter.
This is not all you need to do. You must also drain the tubes around the impeller. The manual explains how to do this. I don't remember off the top of my head. Pretty simple, just unhook some tubes and drain.
I don't use antifreeze.