Overheating engine

[Mr. Sandman]

For coolant to enter the exhaust it sounds to me like the heat exchanger could have rotted thru and is mixed cooling seawater with the antifreeze or you have a head gasket leak of some kind. If the engine is running smoothly then look to the HE. Did you winterize it properly? (Freezing water could crack the brazing inside)
Take it off and either take it apart or have it pressure tested/cleaned/boiled out. Do you keep a temp gun on board? They are really inexpensive and they help diagnose temp problems of all kinds. I keep one in my toolkit. You can measure the inlet and outlets of the HE and see if it is working OK. Also check a lot of other things without taking anything apart.

If the HE is compromised internally, you could be mixing coolant and seawater that will eventually exit out of the exhaust. If the engine is running smoothly otherwise, and you don't see any obvious problems I would look here first.
Best of luck. How hot did she get? Are you sure you have coolant in the exhaust? How much coolant did you loose?