As long as the water is pure you can use it. I have used distilled and filtered tap in the past.
Some people are fanatics about organics, but I confess I have only used organic lemons once, the only visible difference was that they cost more than twice as much and only had half the juice. Since lemons have a thick skin there is not much chance a large amount of pesticides and other toxic elements will affect the flesh. Be sure and wash the lemons before you cut them and regular lemons will work just as well.