MCM 350 MAG MPI
S/N 1A043677

This is my brother's Sea Ray bow rider he bought late last year. He test drove it, had it winterized and then had a gps installed last month. He uncovered it to launch last weekend and no start. I checked it out yesterday and when cranking it sounds like one or two cylinders are firing, but only when I opened the throttle up. I was originally thinking the plugs were drowned with fogging fluid because it sounded flooded. I pulled a few plugs out, they were moist with gas, but not to the point of not firing. When I checked for spark at the plug end, I get maybe one blue spark, a few yellow sparks and then no spark. When I check it at the coil wire, I get solid yellow sparks that begin to weaken. I stop cranking and crank again, no spark. Looks like classic coil failure. I happen to have a complete 99 Suburban 350 on a dolly in the garage that ran fine when pulled. So I plugged in the coil & module (and grounded it to engine). No difference. Swapped the coil wire, crank sensor and the cam sensor from the truck motor, same symptoms. I checked the pink wire on the coil and the module, they both have battery voltage- key on (12.5v) and while cranking (10.2v was lowest I saw). I checked the grey wire on crank sensor and it has 5v, but I didn't check during cranking. It's possible the truck motor's ignition parts are bad (or went bad when I plugged them in), but like I said, exact same symptoms. I followed the new gps wiring and can't see where that has anything to do with it.
Today, thinking about it more, maybe I have a engine ground or sensor ground problem? Anyone experience anything like this? Thoughts?
Edit, I also checked the shift interrupt switch with ohm meter, it is working properly.

I'm an idiot. A metal stud on a plastic intake is not a good spot to check for spark. I was frustrated because it was late and I was working on my boat when I stopped to look at his and was rushing through things.
Anyway, i found a better place for testing spark, it it was good to the plug end of the wires. I had only pulled one plug yesterday, so I pulled out the others and they were pretty gas fouled. Replaced them and it fired right up.

Glad you found the issue and reported fix. :thumbs

A little odd to me that it was put away running, and spark plugs went bad just from sitting. Maybe an excess of unburnt, unleaded fuel sat on the plug tips all winter?