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Honestly, it sounds like a case of clogged carbs. Carb jobs are very common on marine engines. They make up 80+% of all my work. It takes a piece of dirt the size of a piece of sand in there to gum up the works.
The only fix for it is going to be pulling the carbs off, pulling them apart, cleaning them with spray carb cleaner followed by high pressure compressed shop air. Putting it all back together with new carb kits from Mercury, and reinstalling it all.
You will also want to look in the fuel tank if you can. If there is any trash in the fuel tank, it will work itself right back into the carbs and clog them again.
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