The symptoms you describe just do not lead me to think it's a bad coil. All things being equal it's possible but unlikely.
Knowing that you're on last years gas would be suspect. I'd consider replacing the fuel/water separator because this ethanol laced gasoline is prone to attracting water/moisture especially with temperature change. If you did treat the gas when you winterized it's less likely this would be a problem. Really at this point it's almost like the best thing to do is to get the boat into a state where it's running like crap and start troubleshooting whether it's a gas or spark related problem.
Some other things that come to mind
Bad ignition wires, especially the one that you have coming from the coil to the distributor. Even if the wires look good you may have a situation where the insulation has begun to break down and is partially grounding out.
Moisture is getting into the distributor. This condition usually makes the engine run rough all the time though.
If you're carbed you may have a situation where you've got a float that's getting stuck.
Vacuum leak somewhere?
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