A couple of comments from a seaplane pilot.
When the engine is running in a seaplane, the aircraft is moving. When taxiing downwind it may be at the same speed as the wind - you can end up sitting in your exhaust gases. The other main use in Aus for beavers was ag, however I would expect they would have spent most of their ground time (loading) facing into the wind.
I don't recall many pilots leaning the mixture on the water - rich mixture equals a greater percentage of CO in the exhaust gases.
The only times I remember closing the window in the Beaver whilst taxiiing was in rain.