Following a successful sail to Sidney Spit and back after discovering the engine was not discharging water, half way to the Spit.
I think the problem may be either the water pump or the exhaust system. I should probably make myself familiar with both.
Turns out the problem was a big hole in the 'Mixing Elbow' which hangs off the end of the exhaust manifold. Good news is, Gartside found a replacement. Bad news is it's about a thousand bucks.