I also drink about a gallon a day. We have an RO filter at home (we're on a well) but when I'm on a cruise - I just drink the tap water. Haven't died yet - what I do for a week probably won't kill me.
I don't like big ships so it's unlikely I'd ever be looking at NCL anyway, but if they changed the rules on me (if anyone did) right before a trip I had booked under different rules, I'd be pissed too!