Whilst I agree with SD that it would be a huge leap of faith to go to sea with a watermaker as your primary supply, there are affordable, small units around that work on battery and draw as little as 4 amps.
Check this one out:
PUR PowerSurvivor 40E
Of importance to me is that even if the power goes down, it can still be manually operated and the only thing that is of real importance is protecting the membrane from pollution which out at sea is not difficult. At 1.5 gallons per hour, it could produce enough for drinking/cooking for 4 people while you're charging your batteries daily.