BUT IF YOU DO SPRING A LEAK WITHIN THE PUMP SURELY IT WOULD CAUSE DAMAGE INTERNALLY , IT WOUL STILL BE WATER MIXING WITH ELECCTRIC .
Well, with 12volt motors you wouldn't really do any damage electrically, but the water wouldn't do the mechanics of the pump a whole lot of good. Another thing is if you're using de-ionised water, then the water itself will have no conductivity, de-ionised water does no damage to electrical circuits, it's only water with impurities in it, ie tap water or dirty water, that conducts electricity, and the higher the tds in the water the more it conducts.
So if your pump leaks de-ionised water into itself it'll be fine electrically speaking, i cant say if the mechanics would be ok, but the electrics will be fine.
Tim