As promised to Willk, here's a link to a circuit for an adjustable voltage regulator which can be used as a panel lamp dimmer. Dimmer.
The circuit shown as built is more complex than needed for a boat. The LM317T regulator is used, it needs a heatsink of some sort, a tab of aluminium or build it into a wee aluminium box and bolt the regulator to the box. The tab of the regulator is positive so the usual type of mounting/insulation kit should be used. The value of the capacitors is not critical, the one on the input could be .1uF ish and the one on the output side about 0.47uF . Their voltage rating should be higher than they will see on your system. So 25v or higher will be fine. You can't have the voltage rating too high but the components get bigger with increased voltage rating.
Instead of a preset variable resistor for adjustment, a 2k potentiometer needs to be used (R2 on the circuit) (R1 to be 220 ohms.) They come as 'linear' or 'log' for this use it makes no difference which is used. They have 3 connectors, one cable to an end connector and the other to the middle connector. The potentiometer is wired to a piece of 2 core cable long enough to reach the console mounting position from wherever the circuit is mounted. Depending on which potentiometer end connector you use, the lights will dim clockwise or anti-clockwise
When I build mine I mount the components directly on to the regulator's legs so no circuit board is required.
My present one is inside my cabin so no waterproofing is necessary for me but my previous boat was open and I just applied some water resistant grease around the potentiometer spindle and put a greased O ring between the control knob and the dash panel. It worked fine for years without failure. The component cost is small and the bits are available from online suppliers - Rapid Electronics, Farnell etc. or from your local Maplins or hobby electronics store. A very easy build though the weblink makes it look much trickier than it really is.
How to fit it... your panel lamp supply positive cable is cut and connected to the regulator input pin and the feed to the panel lamps is taken from the output pin. A negative is needed too and can be taken from any convenient place on your wiring. I guess I should say that this circuit is for use with conventional filament lamps rather than LEDs.
I hope I've covered everything. I'll be building a circuit next week and I'll add a picture when that's done.