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Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.
Ok we need to check spark when the engine looses power. This can be done with a timing light. Run the boat till it acts up then connect timing light to each spark plug wire watch the flash of the timing light. Do you have a steady flash on all cylinders?
Since carbs have been gone through and the fuel is good that leaves us with an ignition issue.