What you have to remember with Lower Voltages is that the Current is exponentially greater, (Ohms Law.) I.E. a 2Kw Load at 240V = 8.4 Amps. The same load on a 12V circuit = 167 Amps, (a Massive increase). This is one of the reasons why relays and switches are rated differently for AC or DC.
There is also the matter of Arching on switching DC Loads, (higher Current,) For General Lighting Circuits, this should not be a problem, (I assume you will not be using 12V 100 Watt Filaments ). The "Penny Switches" suggested in an earlier post are sufficient for your needs dependent on the number of lights you're controlling per switch.
I would suggest emailing the Reseller/Manufacturer with details of your requirements, detailing the number of lights in the circuit needed to be controlled by each switch and the Voltage.
You stated Individual Lights but; maybe, "banks of lights" would be better unless you want 6-12 switches,(however, you could have both systems together, banked and/or singles. Just don't ask me to draw the Circuits here ).
Maybe, but; there are many situations in a Narrowboat where a 12/24VDC-240VAC-12/24VDC system would be far better than a straight through 12/24DC System, (even given the losses inherent in conversion). One such situation which comes to mind is a battery charger at the stem to recharge the Bow-Thruster Batteries, (Far better than running Very Thick 12/24V Wires Stern to Stem).
I hope this helps.
John