Is there any issur or minhag not to bring food or water to a levaya?
No, there is no problem in this.
However, one should not eat or drink in a cemetery.
See Yoreh De’ah 368:1, and Biur HaGra 1 explaining that one should not eat in a cemetery because this is Kalus Rosh. This will not apply to drinking water if it is hot etc.