I have exactly the same problem with my hallbergrassy 40 and volvo
After heeling a lot or in rough waves my engine will run dry.
Last time even when my engine was running in rough water. I was lucky that I heard the change of the sound and stopped the engine before it ran hot.
Than its not enough to fill the strainer with the seacock closed and opened again when the strainer is filled with water.
even with the system full of water the pump does not start sucking.
I have to open the impellor cover ( 4 screws) and drain the system by gravity. Than it will start pumping again.
I tried new impellors, later changed the water hoses becauses the old had flattened on one place due to the vacuum.
I still have the same problem....
The engine is quite new, only ~500 hours.
Next what I will try when I go back to my boat in April is to lower the Vetus cooling
water strainer which is maybe mounted to high.
But as it is a hallberg-rassy I think they should know how high over the waterline a strainer has to be mounted ?!
..and I will have a look for terra novas tip: "The sea strainer plumbing
must have a continual rise. If there is a trap in it, it the pump will suck air. "
as far as I remember the cooling water passes the gearbox
on its way to the water pump, maybe there is a trap...
or the heat exchanger
When this will not help I try a brandnew volvo
all my former boats were able to fill the empty watersystem just by starting the engine !