It works! It works!

Hallelujah, after changing the gas filter the engine was back to its normal, great running self! So the next time we (or you dear reader) see a filter full of green gunk like this, do change the filter before blaming the carburettor:



For future reference, our carb is Weber type "36DCD14". Thanks also to Pertti for helping to identify the problem, and cleaning the jets in the carburettor. For the future we'll equip the boat with a load of replacement filter (of the see-thru and not opaque kind).