I did mine in the prescribed fashion (every year for all my motors until the first E-Tec - then every 300 hours - which for me at the time was around six months...) then I got cocky and only changed out a water pump when my water pressure gauge showed a drop in performance... that was until.....
One day just after 3:30pm I'm running the Joe River (Flamingo) with customers aboard and know I'm going to be about 30 minutes late back to the ramp.... That, of course was just before my water pump died on me and water pressure went to zero in a heartbeat... Hmm nearly 19 miles to the ramp late in the afternoon with customers aboard... Uh oh.... I make a direct call to Ranger dispatch and start a Ranger my way (and of course that meant a Ranger on the road needed to drive to Flamingo, launch the patrol boat, then run to my location - not looking good for the home team... Roughly 30 minutes later a good samaritan running the Joe route came by and offered to tow us in -- I could have kissed him... Two hours later we got back to the ramp - and to this day I always do the prescribed maintenance interval for my water pumps, period... By the way in that instance, the rubber part of the impeller had sheared off of the pump's shaft - that's why the pump failed. Mechanic said he'd never seen that before....
By the way the rangers will only take people aboard and will not under any circumstances provide a tow... so I had planned on a day's poling - the following day to get home.... not a fun idea at all. I'd planned on having them take off my passengers and leaving me with the skiff...