Get a 20A circuit breaker, wire it in place of fuse...that will save some $ and pains in the neck as you trouble shoot.
look first where harness enters at cowl, lots of movement there. Is it pinched? Peel back the black cover and look for bad wire.
You can also disconnect at the big red plug. Then use a remote start switch. If fuse still blows you are looking at something on the motor side, if it doesn't you are looking at the boat side.
Does operation of the trim switch seem to make it blow? If the TNT switch is on the throttle look at base of throttle handle.
Although Because you state "when I can get it to turn over" I would concentrate on the yellow wire with red stripe and the red wire, both run through the harness.
My 1st guess would still be in the area where it goes into the cowl. 2nd guess would be where harness goes in to boat at the motor well. It doesn't have to be a bare spot with a short. A severely pinched wire can do it also. The pinch increases resistance.
