If by rubber gasket ring, you mean the big one that goes into the bell housing and around the input shaft housing, then you still have a leak to find.
Like Bt Doc said: If there is water in the drive shaft bellows it will pretty much always get into the gear case. The seals on the input shaft are designed to keep oil in. There is no seal there to keep water out since that area is supposed to be dry. Yes, pressure test - but don't consider a little water in the gear oil to be a sign of a bad seal.
You still have to find your leak. Pushed out gasket, bad water passage o ring, bad shift shaft seal, or a bad bellows. Or a case corroded or cracked.
Hope you get it fixed easy.
Rick