Hi Steve,
I think the best place to ask is your local aquatics and fish stores as they will already sell pure water for use in marine tanks. They already will have worked out their margins and you don't have to tell them you are in competition. Although I would keep in mind people who buy more off you may expect a discount, like when you buy leaflets.
Any chance you could have a read if my thread Steve as I think you'd give me a cracking reply?