I have seen the fuel pumps mounted to the seawater pump but I am not that familiar with them.Do they run over an oil bath or is there just grease inside the pump to lube it?
There should not be an open hole on the fuel pump body.A marine mechanical pump should have a hose running up to the air cleaner from that port you are describing.Without it in the event of the fuel pumps diaphragm failing fuel will spray out of the pumps weep hole into the bilge.