I used to have a juicer and would make juice all the time! Boy, I miss it (kiddos got at it once and destroyed it lol!). As for watering it down, it just depends on what you juiced and how it tastes. I never watered down carrot and apple juice, but anything stronger flavored I would add water and brown sugar.
I remember when I first got my juicer and I went crazy and 'juiced' everything! Cucumbers, zuchinni, radishes, ugly fruit, bananas are impossible, and even potatoes! I dunno, just wanted to 'see', you know? For some reason the celery and cucumber juice wasn't too bad, but just about everything else was quite disgusting.
Lettuce makes lettuce flavored water lol! You need a lot of lettuce though lol!
Oh, yeah, spinach wasn't too bad either, but your teeth turn green.
Anyways, hope you have lots of fun and find some really good, yummy juices you like!