For water to get into your gearbox, it must have come past either the propeller shaft seals, the drive shaft seals or the gear selector seal. In rare cases the casing can be cracked from an impact allowing water in.
In order to determine which oil seal is leaking it is best to pressure test the gearbox. This video shows how to do this.
Replacing the propshaft seals the hack way
If the bearing carrier is completely seized into the lower unit you may have no choice but to hack the old oil seals out. This video talks about doing that.
Replacing the driveshaft seals on a Honda BF30
This topic covers replacing the seals around the driveshaft that keep the gearbox oil in and water out.
Replacing the propshaft seals on an Evinrude 150
This video covers removing the bearing carrier and replacing the propshaft seals on an Evinrude 150
Replacing the driveshaft seals on an Evinrude 150
This topic covers replacing the seals around the driveshaft on an Evinrude 150. These seals are responsible for keeping the gearbox oil in and water out.