Slimmy,
It's not the type of job an average mechanic can do without a manual and  
training.
There are actually several seals. One on the top of the drive shaft where  it 
enters the water pump housing and also one on the output shaft for the prop.  
Take the lower unit ( or whole motor) to a dealer and have them replaced. 
Seven  years on a lower unit and seals is not a long time. Something else might 
be  wrong. OR, you simply might have a bad/leaky gasket on the two fill plugs. 
Have them replace the water pump impeller while it's apart.
 
Rummy