I was flying with Ryanair last month and wanted to take some vitamin pills while on board. I went down to the end of the plane and asked for a glass of water, but the stewardess refused. Since the only other option was to buy a 3EUR bottle, I've decided to take the pills later.
Is there some EU law which obligates airlines to provide water while on board? What about if someone needs water to take their medicine?
No, there are no requirements for airlines within the EU to provide free drinking water. I know that it is both common and in some countries even required by law for bars and restaurants to serve tap water for free. If tap water is good enough, I guess that not even Ryanair would have charged you for drinking tap water aboard the plane, but it might not have been very beneficial for your digestion over the next couple of days :)
Your question is however not as strange as it might sound. There are movements within the EU to at least force airports to sell reasonably priced or offer free drinking water in the secure areas. Since fluids were banned in hand luggage, many airports exploit the situation and charge horrendous amounts for bottled water or other drinks in the secure area, making it necessary for EU's consumer protection division to take action.