go with the stage two water injection from snow performance (at least i think its them, the only one they sell on summit)

I want one for my wifes volvo and haven given some serious thought to how to tell when the tank is running low. She's not dumb but being a girl she has a tendency to not pay attention to gauges when she drives. I think the best thing would be to replace the water tank the kit comes with, with a 3 gallon fuel cell. You could easily mount the cell in the trunk without losing too much space, plenty of capacity, and you could use a standard gm fuel level sensor hooked up to a fuel gauge of some sort. You'll always know how much you have so you don't accidently kill your engine. (biggest downfall is the added weight of 3 gallons of water, so you have to remove something else non-essential, like the brake master cylinder or something. )