Sorry, but Larry's the closest to getting paid so far.
Squeezed in enough time to zip out to the boat, and the hot water tank hoses are above the shut offs to the pilothouse heater. (They were closed when I drained the engine).
Larry logically suggested to open petcocks to see if any coolant came out. The starboard petcock didn't, which drains the cylinder head and cylinder body.
But...(insert D'oh! here)...I just realized I didn't open the coolant filling cap when I opened the petcock, so guess I'll be heading back to the boat after work to narrow down the diagnosis.
Life at the bottom of the learning curve