The A4 raw water pump rarely fails. Check the impeller, the flow to the pump when you have the cover off the water pump (may indicate an issue with the external strainer or the seacock and associated hoses), and the internal water strainer.
If you haven't checked it yet, my money is on the impeller. Get a new impeller and gasket from Moyer Marine first - replacing the impeller annually is normal maintenance, so you might as well do it when the cover is off. Before you take the impeller out, be sure to photograph it or draw a diagram to show which way the vanes are oriented.
Just so you know what you should be looking at, the photos below are of an impeller in a water pump with the cover off and the straining basket of a water filter. They're from a Yanmar diesel, but the A4 parts will look the same.