If the sole of your cockpit is above the waterline then water should not come up through the rudder post tube. I would check and make sure that when the cockpit is full of people, the floor of the cockpit is above the water line. This assumes that the rudder post tube is at least as high as the floor of the cockpit.
I had a loose and wobbly rudder post/tube on one of my boats and I was told the quick fix is to make shims out of the sides of plastic milk bottles and slide them in until the tube can be rebuilt. It worked a charm!
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217