Assuming the motor runs on all 6 cylinders, and the prop is the right pitch, the carbs are the next most likely issue.
The carbs on those motors must be set on the water, motor warm, under load. If all the rest of the link and synch is good, you likely need to richen the carbs a bit to allow smooth, fast acceleration. I recommend a fresh 50::1 gas/oil fuel supply, for setting the carbs. Start with all carbs 1-1/2 turns open.
So warm the motor on the water, in gear at idle. Open the mixture screw (richen the mixture) on the top carb 1/8 turn and try to accelerate. She will still likely still stumble. Open the carb on the mid carb 1/8 turn and try to accelerate. Repeat with bottom carb, and then repeat sequence starting with top carb and testing acceleration after each adjustment. You may need to run thru sequence 3 or 4 times to get it right. Listening carefully to the engine accelerate can help. You can find the bottom carb will be 1/8 turn open, the mid carb 1/4 turn open and the top carb 1 turn open from initial setting, or something like this.
Generally top carb will need to be the richest, followed by the mid carb and the bottom carb will be set the leanest. Remember the richer the idle mixture the worse the idle, so you want a compromise that gives good acceleration and reasonable idle.