What are you trying to accomplish?
If you are looking for fuel economy gains, it's snake oil. Electrolysis to split hydrogen from water molecules takes more energy to accomplish than burning the resulting hydrogen releases even with the absolute purest water. Any contaminants in the water just makes it even less efficient.
Think of it this way: The military would have adopted it years ago if it worked. The logistics of fueling is a major headache, especially in times of fast operational tempo. Hydrogen requires a lot of electricity up front to work, and if you're getting that from your alternator the extra drag is using more gas than you're saving burning the resulting hydrogen.