Check boarding ladder bolts. Mine leaked there and I tightened them and it stopped for a while. I still get some. If it were the holding tank drain I think you would know it. Could also be where the rub rail bolts are. Need to see if it is fresh water or salt. Mine is fresh. I think mine could be coming from where the screws that hold the plate where you stand at the wheel go through. I need to seal those up.