Gotta throw my hat in on this one. If a head gasket blows, you can get water/coolant in the engine oil, which is going to lead to inadequate bearing lubrication, metal shavings chips and grunge getting into things, cylinders scored, that sort of thing. If you have a bearing starting to fail, the whole thing compounds itself. Would not really have anything to do with fresh, salt or brackish water for cooling. It could be really be directly a result of a head gasket failure. Soungs more like you had scored cylinders than corroded cylinder surfaces.
I have followed you many posts about how happy you are with your skiff and the 100 hp outboard, but if you were underpowered or "just enough" powered, there would be many times when your engine was lugging/pulling too hard. Bet you skiff stipped down weighs 1500 pounds, add gas, outboard, gear, two or three guys, maybe a hull built more for displacement and stability than easy planing (just thinking out loud, not sure...), that cylinder head was saying enough is enough, I am outta here!
Could be you got really good life out of a motor that was just a bit too small for your application. Sort of like tow vehicles, dont tow with something that is just big enough.
I have the 115 Yamaha on a 19 foot Scout. You will like it as much or more than the motor you had.