I had the exact same question as you many moons ago.
apparently, your 2A 250vac switch is good for 4 amps if you half the voltage to 125vac... good for 8 amps at 60vac... good for 16A at 30vac... and so on.
Of course, we're talking AC here - not DC. But, for the most part, pretty much stands good for model boaters' and their DC supplies anyway.
Depends on how many amps your system is pulling. As Malcomlfrary sez, 5A 30V sounds a bit puny.
Maplin might be able to help you with the waterproof covers. You could also try technobots.co.uk