Typically there are two seals
, one to keep the oil in and another to keep the water out. The seals
in one direction.
Often the water seal in the lower unit will be compromised, allowing water to come into contact with the inner oil seal
and as the unit cools after being run, water is drawn in because of the contraction of the air and oil contained in the outdrive. No oil will leak out, but water will 'leak' in.