If the gear lube is not in the bilge or leaking into the water it may be the upper seal and will leak into the drive shaft bellows. If you remove the drive and there is lube inside the bellows it's the upper seal. If no lube here drain the drive and perform a pressure test. Typically a leak is not dectable in the water because it mainly happens when the prop shaft is turning at high speed. Any sign of water in the lube if you remove the drain plug?
1997 2400 SCR
5.7 Vortec / Bravo 2