When you say you checked for spark. I guess you mean at the plug, as you can't check it at the coil output. First, with a volt meter, check for 12v at the plug that goes to the cap/dist. If you have voltage that far all your ignition circuit to there is okay. The next thing I would suspect is the module, those hei deals can just quit like turning off a light switch, (which is why I would never put one in a boat). You should pull the cap and rotor, look under the center of the rotor to make sure you're not grounding straight through the rotor to the shaft. If that is the case there will be a trace or burn mark under the center of the rotor. If you didn't have 12v at the dist, hot wire it directly to the batt + and see if it will fire. Don't leave it hooked up while not running, and don't run it long if it does start. (I don't know how your system is wired, and you could damage components wiring directly). If the motor will not come around, consider changing the ignition system to something very simple, non hei, with standard coil, dist, and either points or a magnetic breaker deal. Those systems can be troubleshot out on the water in the event of a rare failure....MP
PS make sure there is no moisture in the cap from condensation after the last time out.......