In a nerdy, don't really need to know way, just correcting what seems to be a general misconception, though there are exceptions...
It is likely that there is no ram effect at the water intakes on the side of the gearcase when the gearcase is moving, it is likely that there is a pressure reduction at the water intakes on the side of the gearcase.
When a fluid is accelerated the pressure within it falls. The water around the gearcase is accelerated because the gearcase takes up the space where water was just a moment earlier. If there is to be no vacuum behind the gearcase the water has to get there quickly, the distance around the gearcase is further than the direct line so the water needs to speed up to get to the rear of the case. Hence a pressure drop within that water.
It's the principle used to provide forward movement from your propeller, to provide flight, to draw fuel from a carburettor etc.
Worth noting is that if you bring your boat close to an object while moving; another boat, jetty etc., the water between the boat and object will be accelerated so your boat will be drawn closer to the object because of the reduction in pressure in the water in-between. Sharp folk will notice that when coming close to a stationary object the water does not initially appear to be moving but it will be forced to by the presence of your moving hull. An object moving through the water has the same effect as water flowing past a stationary object.
Two boats coming alongside each other when moving will be drawn together quite powerfully.
Does the impeller need to be primed? Not in my experience but that is limited. It should be a tight fit inside the housing and capable of pumping air. And, to link with a previous thread, a drop of lubricant is a good idea on a new instal to prevent the grippy rubber running dry.
All IMHO, of course...