Is there water in your lower unit gearcase oil? Water intrusion would make the gear lube look milky, and would quickly wear out your gears. The most common and hidden area of water intrusion is the seal around the driveshaft or the o-ring, both located as part of the water pump base. 
 
A potential easy fix could be changing out the base of your water pump.  Our Water Pump Base kit includes a new Base, Seal, O-Ring, and all in a Kit to make it easy.
 
Part numbers: 46-42579A 
 
Contact Mike Hill at STVracer@yahoo.com with any questions.