When hooked directly to your battery, have someone turn the key to the cranking position and check for voltage at the starter solenoid with your multimeter by placing one probe on ground, and the other on the terminals of the solenoids. What does it read? If you access the back of the ignition switch is there a voltage differential between the switch and the solenoid greater than 10%? Is there a voltage differential greater than 10% between the battery and the supply to the switch? If you run a wire directly from the battery to the small positive terminal on the solenoid, does the motor now crank? Can you turn the motor over by using the emergency pull cord? Are there any signs that rodents have been in the boat, or under the engine cowling?
If all of the fuses are good under the cowling, and you have low or no voltage, suspect the cables, harness and ignition switch. Begin by inspecting the heavy cables... Any nicks in the insulation? Bulges due to corrosion? Are the cable ends corroded, loose? Is there green dust? Is the cable hot due to resistance?
The solenoid should have an audible click when the key is turned, if you hear the click, but no action from the starter, suspect the starter, but remove the heavy gauge cables, negative and positive from the motor end, wire brush them and apply dielectric grease before jumping to conclusions.. If there is no click from the solenoid, and you have 12v+ at the solenoid... Time for a new solenoid.