J and S,
Just to clarify...most of the restaurants, including the main dining room are included in your cruise fare. They offer free iced tea, lemonade and water, so it is very possible that you can go on your cruise and eat as much great food as you want and not pay anything extra, other than the gratuities which are generally given on the last night of your cruise. So, for example, if you eat in the buffet for lunch, you do not have to leave any sort of gratuity for the persons picking up your dishes. You just finish eating and walk away.
If you go to a specialty restaurant, you will know right away if you are being charged and there should not be any surprises.
They do charge for any soft drinks (sodas, e.g. Coke/Pepsi) and beer, wine and liquor and again, you will know how much it costs at the time it is delivered, or you can ask before ordering.
MCPayne posted a great link to explain the gratuities, so check that out as well.