Phil T wrote:....the heat from the lamp would evaporate the moisture in under the bezel.
The method Phil T suggests is a useful method if the particular gauge has illumination and if that illumination is provided by lamps that use incandescent lightbulbs.
Incandescent lightbulbs generate proportionally more heat energy than light energy, so wiring the gauge illumination circuit to operate from the engine ACCESSORY circuit will result in those incandescent lightbulbs generating heat inside the gauge housing whenever the engine ignition key switch is in the RUN setting.
Modern outboard engine gauges may not use incandescent lightbulbs to illuminate the gauge, but instead will use light-emiting diode (LED) devices. An LED does not produce very much heat output (compared to an incandescent lightbulb), so trying to warm the interior of the gauges by re-wiring the gauge illumination circuit won't accomplish as much heat generation as would occur with incandescent lightbulbs.
Also, if the gauges use a liquid crystal display (LCD) the gauges will have a backlight that is already operating all the time, and there may not be any separate gauge illumination circuit to be energized.