[C320-list] Coolant mixing with raw water

[Marianne and Kevin Waterbury]

When we launched in the spring, I noticed a milky color to the exhaust water.  We motored to our slip where I decided to run the motor till the water came out clear.  Instead the temp of the motor went up and I shut it down. When it cooled I opened the radiator cap to discover no antifreeze in the radiator.  I filled it up, and ran it again.  The next day I checked and no antifreeze.  Hoping it was a bad seal on the rubber end caps for the heat exchanger, I replaced both with new caps and ran the motor, same result, coolant was gone.  The only antifreeze I found in the boat was what I spilled while adding it.  The oil color is fine. How is my coolant disappearing, has anyone run into this problem?  
Kevin Waterbury Hull# 111