Could you explain how you know you are not getting spark? Do you know what your fuel pressure is up to the fuel rail? From your explaination, you always seem to have trouble when the boat is warm/hot, after use. Do you have any issues when the boat is cool.

You said it started doing it more (in the beginning) under load. I would suspect more of a fuel issue. The boats are known for vapor lock, fuel pump, and filter issues. That is why I am leaning to that side.

Also, I had to replace my ignition switch because it developed a dead spot on the keyed "on" position. I thought it was a fuel/ignition issue. It was cheap, you may want to try there also.