After purchasing the Cape Cruiser 3 years ago, I spent several months going through the boat and related systems. Some of the work done, was to update systems, some was to fix issues and a lot of it, was general maintenance.
There was not a lot of fuel in the tanks, so I drained them, flushed lines and changed all filters. Flushed and changed oil and filter and inspected and pressure tested the lower unit, then replaced it's oil. Replaced thermostats, water pump impeller and removed the side mid section cowling, to gain access to the tell tail hose, as it was clogged. I placed a water hose into the thermostat opening and with the lower unit off, flushed about a cup of ground up oyster shell material, out of the engine. There was zero sign of corrosion around the thermostat openings and none around the lower unit, or the mid section interior.
So, without pulling heads, there was no other sign of issues... It is apparent that all of this internal corrosion happened on the previous watch. I am fortunate, as I really got the boat all-up, for a bargain and I did get 400 hours on the engine, with no other issues.
Regarding corrosion and Honda's, I do work on a lot of outboards and almost every time that I have seen corrosion, it is because the outboard was not flushed after each use. I have seen severe corrosion in every brand, so they all have their issues, if not well maintained.