Get a DPDT configuration on your switches. Preferably illuminated rocker-type switch. You can get those at most marine stores. Mine cost 20$ CAN
While your current set-up may not neccesarily require DPDT at this time, you'll be happy you have it if you upgrade your system later on. For example, in a good automatic multiple ballast system you can typically have a single fill pump for multiple ballast bags, but separate empty pumps, one for each bacg (total 4 pumps). In that case, you'll need electric solenoid switches to control fill flow to desired bags to fill. DPDT switch will allow you to do that. Otherwise, you'll have to mess with getting relays and that gets compicated as hell.
I just finished a nice project of this sort. I'll post more about it when I'm less lazy...