Yes a bad prop seal or upper drive shaft seal can cause this type of loss. No signs of lube in the bilge?
If it is the prop seal there is the potential that sea water is getting sucked into the drive as well and this can destroy bearing and gears. With the boat out of water drain the lube and perform a pressure test.
1997 2400 SCR
5.7 Vortec / Bravo 2