Your nav lights work because they are powered through the "boat harness" not the "engine harness". You said you checked the fuses at the engine. You checked them how? Since the starter and trim both are inoperative that says power is likely interrupted between the battery and the engine. Although you say connections "llook" clean, the only way to verify this is to disconnect the battery cables at the battery, make them shiny clean and then tighten them securely. Now repeat the process on the other end of these cables. Be sure to inspect the cables where they enter the cable ends. Lumps and bumps under the insulation is an indication of hidden corrosion that can interrupt current flow. If this fails to fix the problem it is time to break out the voltmeter and follow the juice. Current must pass from the battery, up the engine harness, through the ignition switch, back to the starter solenoid and then to the starter. I doubt the problem is in that circuit because the trim doesn't work either. The problem in my view is under the engine cover as a kill switch or neutral start switch will not cause the trim to not work.
