I just had this problem. It was a slowly leaking "rad" cap atop the heat exchanger. It was rated at 14 PSI: I got the same sized one for nine bucks at an auto parts store rated at 16 PSI. I'm glad I did (see below).
I use green "diesel coolant" at 100% strength. I must be doing something right, because my V-belt snapped when I was puttering to and from the poop dock, I didn't notice and the non-circulating coolant got to 230 F without boiling over or otherwise exploding.
But I sure as hell now know why they call it an "expansion" tank.
There are drain plugs usually on the front of the heat exchanger and/or at low points on the block in the cooling circuit. I have the shop manual for my engine (Westerbeke W-52) and I must say I refer to the thing about twice a month just to get used to all the extra crap on a bigger diesel when compared to an Atomic 4...it's a hell of a lot more engine.