onegai shimasu vs. kudasai

[ginam]

Can someone please sort these out for me? I don't want to seem like a hayseed with no manners, nor like some bossy American. But I don't want to fumble on any politeness words either...I'd like to be respectful. This mainly comes up for me in restaurants, although I would like to know about it in general. I don't think these are interchangeable, or are they? Someone said they have to do with 'please give me' or 'please do for me,' but it was either a travel book or another foreigner.

When I'm shopping, I just nod & smile & pay, but in restaurants I'm actually requesting something. Since I'm still new to Japanese, I'm almost exclusively pointing to pictures or something I recognize. Should I add a 'please' to this too, and if so, how?

The other day, I figured out that someone working asked if we wanted her to bring us a pitcher of water (one of the services, the tea was on the table already). In the States, I would have said, "Yes, please." What could I have said here?

Thank you in advance.

 

[undrentide]

I think that "onegai shimasu" (meaning "please") is the best and safest.
Since it's a simple phrase, it would fit in most situation.

"kudasai" is polite enough when ordering for something at a restaurant, but this phrase you can use only for ask someone to give something to you.
"Onegai shimasu" can almost always replace "kudasai", but not the other way round.