I change the gear lube in the lower unit every 6 months. Sunday when it was due again it drained nothing but water, not water mixed with oil, just water.  There were no warning signs, no leaks, no fishing line wrapped around the prop shaft.  My theory is, it just gave up after 7 years (1998 motor).  This might seem early but I fish all 12 months almost every weekend. I caught it early and only ruined the pinion. 
I post this as a reminder to all, at least crack the drain plug every couple of months and make sure no water is present.
When you change lube also change the plug gaskets or o-rings.