Water leak/mixing with oil

The gasket on the gear shaft is dead. Water runs around it between the outer cover and the gearbox, and unfortunately the gear shaft needs to go all the way through them to get to the 'box. There should be a flat O-shaped gasket around the shaft, and that's leaking water into the oil, and out the side. This is a pretty tough job if you havn't got the right tools, so maybe rather take it to a workshop mate! Hope that helps!