You say Starboard bilge pump? As far as I know the 25 SC has two pumps , stern (below the prop shaft) and port ( near alternator. The pump on the starboard side lazzarette is the domestic water. Assuming it is a bilge pump pumping, is it fresh water or salt water? You say the bilge is dry? Open your hatch and see if the water is rising before the pump engages and see if it tastes salty or fresh. Fresh water could be a hose connection on the domestic water. Usually your domestic water pump would be cycling as well. Is your air conditioner running? That adds fresh water to the bilge. Salt water could be your packing on the shaft as first suspect. 5 gallons every 20 minutes is a lot. Check your exhaust fitting and check both thruster compartments. The rear one does tend to have water in it, but see if it is and excessive amount coming in.