Drop down pressure test to see where it is leaking from. Could be a seal on the driveshaft end or the prop shaft end. Could be the filler/drain screw gaskets or a slightly bent prop shaft. I just rebuilt my 9.9 Yamaha lower end with all new seals throughout, replaced the prop shaft as it had a wear groove where the outer seal rode on the spinning shaft. The driveshaft had the same type of groove from the seals...that part is now obsolete and would have been over $400 if still available, Took it to Clarke Engineering and they spray welded and polished out the grooves for $175.