The brick on most computers will output 15 VDC ( or whatever voltage is required ) from any input voltage between 85 VAC and 220 VAC. Your dimmer needs to be on the other side of the brick between the brick and voltmeter. A simple rheostat resistor for DC is the dimmer you want.
Are you sure the brick is functional ? ?

You said the “brick” is a transformer & rectifier so its output is 15 vdc. Wall dimmers for lights are usually triacs, which operates with AC only. So a dimmer won’t work on the output side. The brick’s input is expecting ordinary AC from the outlet, not clipped & chopped by a dimmer, so its behavior plugged in to a dimmer would be erratic and maybe have a tendency to act all-on or all-off.

What you need is a potentiometer (aka “pot”), which is an adjustable resistor, on the output side to control the DC voltage into the voltmeter. Salvage a volume control from any junked radio or tv, or spend a couple of bucks at Radio Shack. You might have to add a series resistor to the pot, however, so turning the knob all the way to one end doesn’t short out your power supply. You won’t like the smell of burning plastic!

I don’t know anything about boat lights, but an incandescent bulb should not be over-voltaged. So if it’s rated for 12 v I’d worry about 15 v causing it burn out early. Otherwise I’d choose the lowest amperage / power / brightness. A light bulb is a big load for a small wall wart. (Fortunately the meter adds a negligible load.)

Computer power supplies are switching-mode regulated. Although a dimmer switch reduces the AC voltage, the computer PSU is going to try to compensate for that. A dimmer is what we call a diac/triac circuit that reduces the AC voltage by clipping. This is not suitable to your application. If all you’re interested in is watching the needle move while you twiddle the knob, just set the voltmeter in AC mode and put it across the load. You can use an ordinary light bulb for that. If you need to reduce the voltage, just go to Radio Hack and buy a 12v transformer. If you have a DC only meter, buy a diode while you are there. And pick up a book on basic electronics. Very basic electronics.