Let us know how all that turns out. If you are not cranking at all, then you need to go on a bug hunt.
The fuse may look good, but pull it and test it for continuity across the tabs. Clean the fuse tabs and the female tab receptical. Check that the wires going to the fuse are solid and well connected.
Next, make sure you have little or no resistance between the battery ground and the engine block.
Then, pull the plug where your control harness from your helm plugs into the ECM - inspect and clean.
Then run a wire from your battery positive directly to the big positive terminal on your starter, bypassing the solonoid. If the starter cranks the engine, you have a problem between the key switch and the solonoid. Next, touch the positive wire from the battery to the small tab on the starter. If you hear a loud click, the solonoid is probably good.
Then start with the key switch and test that circuit. Make sure you are getting 12V to the switch and work backwards.
Not sure what kind of key switch set up you have, but if you can get to the back of the key switch, make sure the 12 volts is being transferred to the "on" and "start" terminals when you turn the switch. Then test the same wires at the end of the harness plug at the engine.