I don't know what steps you have already taken to find the problem. Starting at the beginning, the coolant must be going somewhere and there are only three possibilities. The oil (which is easy to see if there is any long term noticeable leak), the bilge (which should be easy to see but can be hard to find the cause), or, by process of elimination, out the exhaust. If you are sure it is going out the exhaust, then the leak could be cracked block, cracked head, bad head gasket, leaking coolant from the exhaust manifold or a leak in the heat exchanger (and maybe some other I have not thought about). The coolant portion of the heat exchanger is all copper, has no gaskets (all soldered), and is well protected by the design. So everything on a boat can fail but this should not be a likely candidate. If pressure testing seems to be called for whatever is to be done (sealing off hose fittings, installing an air valve and a pressure gage) could be done without removing the heat exchanger from the boat. In my experience, taking stuff like that apart and off can lead to other issues that take more time to resolve than the original test. Also, I have tried to repair a heat exchanger that had the outlet to the dripless prop seal cracked off. Can't be done. The copper pulls the heat out so fast the brazing is hopeless. Since raw water pressure at that point is less than 3 psi, I did it mechanically with a threaded connection and some rubber and steel washers. If those internal tubes were cracked I think a new unit would be your only choice.
For what it is worth, look hard at the exhaust manifold. In my case it had rusted through after 18 years and developed a pinhole leak that, under pressure after running to temperature would dribble coolant into the exhaust stream. It also dribbled some into the cylinder after shutdown and I saw it with cheap camera that I could stick into the spark plug hole. Of course knowing that the exhaust manifolds "never" fail, I pulled the head and rebuilt that side of the motor. Back together and it still leaked coolant! Pulled the exhaust manifold, figured out how to pressure test, and found it leaked. Replaced it and all was good. Until 4 months later when the leak came back. Classic amateur mistake (always do both sides) since now the leak was on the other side! Changed that exhaust manifold and finally all was good.
Another tip that is sometimes useful is to get from GM the florescent tracer they sell to put in the coolant, Combined with a UV flashlight this can confirm (at night) just where the coolant is going.
Good luck!
CJ
