HarleyQuinn,
On my 2014 R31S, I had a seepage of sea water at the D4-300 exhaust port through the hull. I first noticed it due to a streak of rust colored stain coming from the bottom of the exhaust port along the inside of the hull. That stain was always wet, even after I wiped it dry a number of times over several days/weeks.
At the 2017 Ranger Tugs Rendezvous in Florida, the factory technician thought the screws on the flange on the outside of the hull needed to be tightened. That that did not help. Next, where the exhaust hose clamps to the exhaust port, I then replaced the lesser quality SS clamps with high quality T-bolt SS hose clamps - hoping that would stop the seepage. T-bolt hose clamps permit extreme tightening without damaging the hose. No luck!
It appeared that the exhaust port was seeping sea water due to crevice corrosion on the stainless steel exhaust port. There was no bonding wire to this exhaust port even though it is always partially submerged in sea water.
So, while the boat was out of the water for Florida hurricane season storage, I had the entire exhaust port through-hull assembly replaced with a new one. At the same time, I had a bonding wire attached to that port and to the nearby generator exhaust port, with both connected back to the common bonding at the stern.
After over 3-1/2 months in sea water since the repair, there is no sign of any seepage. I suggest that everyone periodically examine that area for any leak or seepage. Also, you might consider bonding that assembly if it is not already done.