I think in my social environment, a cash bar would be considered rude. I have to say, though, that I can't consider providing water only rude, but I would say it's poor hosting. When I attend BIL and SIL's reception, they had a cocktail hour that only provided beer, wine or water. Apparently, one branch of BIL and DH's family was not satisfied with that option (for one, the kids didn't want just water, along with those family members who don't drink alcohol) so they left the hotel where the reception was held to run down to the liquor store nearby and get non-alcoholic beverages for all of them. About an hour or so later when the HC and wedding party arrives, I saw BIL putting out soda cans from 12-packs (which, to me, indicated poor execution resulting in unintentional poor hosting for the first 1.5 hours). However, it was clear looking at our table (I sat with that branch of the family) that outside beverages had been purchased (I stuck with water when I arrived, I think). We didn't have hard liquor at our reception (couldn't afford it), but we did MAKE SURE TO BUDGET for beer, wine, sparkling wine & sparkling cider (for toasting), bottled water and three soda options (regular Coke, Diet Coke and Sprite for those not wanting caffeine). We also had a coffee maker (but it was broken so, not by our fault, we ended up not having coffee -- we had lots of everything else so our guests didn't care). Anyway, the point is that we BUDGETED to have beverage options, not provided a cash bar so that our guests had to buy anything other than water (and, for OP's situation, was that bottled water available or just water from the tap?). Again, water only is not rude, in my opinion, but for a formal event like a wedding reception, I think it's poor hosting (very rude, though of the bride to not only expect a gift but call out OP on it). And, honestly, non-alcoholic options really aren't expensive so it makes me wonder on what the HC spent their wedding budget (the only scenario I would think water-only is an acceptable option is if this were a reception only serving cake).