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Hi and thank you for your question.
The symptom you described sounds like you are losing spark. We need to determine exactly what it is before moving into why. I suggest connecting an automotive inductive timing light and take the boat out and re-create the conditions that cause it to quit. The timing light will flash when you are getting spark, and if you lose spark, it will quit flashing. If the flashing continues as the engine dies, then it is likely a fuel problem. Let me know what you find.