I haven't had much time to look into this yet, but after searching these threads I haven't see another problem that matches my symptoms.

When the boat sits, the antifreeze in the reservoir slowly goes down. Not to empty, but almost. When I start the motor, I get antifreeze mixed with the water coming out the exhaust. It is only for a few seconds then appears to run clean. I haven't noticed any decrease in antifreeze when running (although haven't been able to run any more than idle), and the oil is clean.

Is this a head gasket or heat exchanger? I am leaning toward heat exchanger since these are known culprits, yet most threads mention their reservoir filling. My initial thought is since I've only been able to idle the motor, the 2-5PSI of seawater pump pressure is less than system pressure....thoughts?

remove the end caps off the heat exchanger,,,with that much antifreeze leaking,,you shoul be able to tell

I'll give it a look see tonight...I was thinking this morning as I was laying in bed that since I had antifreeze coming from both tail pipes the problem points toward the heat exchanger. Thoughts?

id take the heat exchanger off and let a rad shop look at it,,,its a good time to clean the exchanger out also

Just went through this process. I first started with the coolant level dropping and then the overheat alarm came on. I could not find a problem so I replaced the seals for the heat exchanger and put it back together. I pressure tested the system and it would hold pressure. Put coolant back into the system and took it out. When I got back, the coolant was blowing out the tank and the tank was full of this brown milky sludge. I had a hard time beleiving it was the heat exchanger and not a head gasket. I replaced the heat exchanger and it looks like the problem is solved.

If you find coolant in your bilge after the boat is stored , especially in cold weather, I would recommend replacing the two seals under the heat exchanger that the exchanger tank plugs into on the crossover manifold. this is a common problem and the seals are easy to change and relativly inexpensive.

Best Regards,
Ray @ Raylar

All good points...wish i had seen coolant in the bilge, would certainly make the trouble shooting a bit easier. While on the subject, does anyone still use the red coolant (dex cool or something to that effect)?

Thats what I use. - Premix

Thought I would add an update for anyone loosing coolant. As suspected one of the tubes at one end was leaking at the separator plate. The location of the leak was pretty much under the brass end piece which meant that had to be removed before the repair to the tube could be preformed. The service guy soldered the one tube shut, pressure tested to 25 PSI, then soldered the brass end piece back on for $160.00 out the door. Hopefully this won't turn into a case of throwing good money after bad.

the image below is what little you can see of the repair (taken with my cell phone).