I am going to say that this is not true. I study science at university and this violates the first law of thermodynamics; which states energy can neither be created nor destroyed and in an isolated system the total energy is always the same.
Water is a molecule of H2O, Hydrogen H2 releases energy when it is reacted with oxygen to form water (H2O). So the problem here is that it is a circular reaction.
2 H2O + Energy --> 2 H2 + O2
which then reacts to
2 H2 + O2 --> 2 H2O + Energy.
You neither gain nor lose energy through this reaction and that is even if it does occur so that pretty much is theoretical proof that this device is a scam.
If you want to increase you're fuel efficiency the best way to do it is to work with your car. For every unit of gas burned you produce 1 unit of energy say. You want to make this unit of energy count for as much as possible. Ways to do this are simply making adjustments to your driving habits. This is especially important for long distance trips like going up north.
1. Use the gas and break pedal as little as possible. Accelerating very quickly, like flooring it after a green light is very inefficient. You put the car into high RPM and while you gain speed faster you burn more energy getting there. So if you are at a red light be light on the gas pedal and get to cruising speed fairly slowly, you want all the gear changes to be done at as low as possible RPM. The break pedal is the other one; you want to use this as little as possible, every time you break you are losing potential coasting energy which is burnt fuel. This doesn't mean disconnect your break and just pray for the best it means avoid the style of driving where you gas up to 130 get near the car ahead of you break down to 100 then gas back up to 130.
2. Speed is a bit more tricky. Every car has it's own ideal speed depending on a ton of factors, where the gears are set, how much HP the engine has, the aerodynamics of the car. What you want is to be in the very lowest RPM of your top gear. For my 03' Accord this is around 90 km/h or give or take but I still drive 100 on the highways just to save some time. Pretty much you should not be going above the speed limit, yes you save time but you burn more fuel. One of the few cases where time is not money. Also connected with point 1. is that you want to keep a constant speed, if you are going 100 go 100, not jumping back and forth between 95 and 105. Cruise control is ideal in this situation.
3. Take care of your car. Keep up to date with your oil change, make sure your tires are fully inflated, have a mechanic check under the hood that everything is in good order.
So to sum up, take your time driving, go the speed limit and try to cruise control and if you don't have that keep a steady foot to keep a steady pace. Fishing is about enjoying yourself, enjoy the drive up and the scenery as well. Save green by enjoying the greens.
Cheers