Is it okay to substitute lemon juice from a bottle from regular slices of lemon for lemon water?!
Answers:
Actually, "water with lemon" (like at some restaurants, etc) is flavored by the oils in the skin of the lemon, not by the juice (which is sour).
So if you want the real thing, you wouldn't get it by using lemon juice (from a lemon or from a bottle). You get it by just dropping a (non-squeezed) slice of lemon into iced water.
If you like that highly-diluted lemon juice though, you could use either kind of lemon juice to get it.
P.S. You can freeze any part of a lemon (in a baggie) that you don't use for one glass of water, then use the rest of the lemon later to save money.
Why not? It's your choice. But while I'm at it you should know that studies have show the lemons, etc., used when you ask for water with lemon at a bar or restaurant are the filthiest things there. Everyone handles them and no one thinks about cleanliness once they're in the container.
If you don't mind the taste, however, I think lemon juice out of a jar/bottle definitely has a more acerbic taste and tastes less lemony than real lemons. If this is for guests, I would use real lemons.