There are lots of places for water intrusion. The shift-shaft has a seal on top that can leak. But this is a rare point of failure. Ive had a few but I run with no exhaust liner which lets that seal encounter a lot of hot exhaust gasses. I replace it yearly as a result as it will get hard pretty quick. In a stock motor, this isnt an issue. There is a seal on the drive shaft below the water pump. That can fail, although again that is unusual. The two most common points of water intrusion are either around the propshaft, where you get it gritty or else suck in fishing line which will work under the seal and break the water-tightness, or around the big o-ring where the bearing carrier seals against the lower unit housing. If the cover nut (that 4″+ inside gadget that holds the bearing carrier in) cracks, or if the threads wear out, then the cover nut will back/pop out a bit, and that breaks the compression seal the o-ring requires to keep grease in and water out. Since they replaced the lower unit completely, this is an interesting problem. You now know that it is unlikely to be a hardware-induced failure having gone through two completely different lower units (they did replace the complete unit I assume, not transferring the propshaft or driveshaft from the old to the new?) this leaves an environmental issue as the only real possibility. That is, how/where you are operating the thing has to be the issue at hand. Whether it be fishing line, sand/grit, a badly out-of-balance prop is unknown, but any of those can lead to this kind of failure.2008 Pantera Classic2014 Mercury Pro XS 200
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