First and formost go to iboats.com or comparable and get a manual. Willl save you LOTS of money in the long run. You say the plugs look fine. But have you tested spark?? You can get a single spark tester at the parts house for cheap. It takes longer than the multiple testers because you have to do one at a time. See what the spark looks like.....and note the color. Should be bright and strong; white to bluish white....not yellow. I would also trace all my wires paying special attention to the kill switch. Many times this get a loose contact...they are spring loaded...it will run for a bit then die. As already stated trace everything and check close for corrosion. I agree with the above though it is probably electrical. But those are the hardest to find. If you get a manual go through the coil, stator, powerpack, voltage regulator tests.
"But if serving the Lord seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the Lord.” Joshua 25:15