your keyswitch is going bad... or even the key itself is worn... Mine does that if I'm using my older key. The motor can be running but the key is just ever so slightly off of the run position (there is no real accessory position) so the gauges are powered off. Remember your instruments are part of the engine harness and are separate from the boat wiring. Also outboards ignition is magneto based so they only shut down when the key is switched fully to the off position which closes the magneto circuit to ground and shuts the motor down. When this happens to you just turn the key slightly to the right into the run position (being careful no to turn it to the start position) and your gauges should come on. Your key should have a number on it. Just get that number and go buy a new one from your dealer or order one online. Also don't have a bunch of junk on your key ring. The weight of all that stuff hanging from it will damage the switch over time.
