Personally I tend to be more concerned that no security checkpoints are operated downstream of the last opportunity to replenish water bottles before boarding the plane.
If there are any such cases, a lack of bathrooms would seem to be a much bigger problem than a lack of water.
There are indeed such cases. A number of airports (Kansas City International, Zürich Kloten, Amsterdam Schiphol, and others) have long operated their primary security checkpoints at the gate rather than in areas reasonably adjacent to the check-in desks. The limit on liquids (which was initially a ban) was introduced in the summer of 2006 and I know from personal experience that Schiphol was still operating gate-based checkpoints in the autumn of 2007. I would expect the operators of all airports that still operate such checkpoints to be transitioning away from them as fast as they can, but this will not be an overnight process for most of them, especially in cases involving new capital spending.
Also, a number of airports now operate secondary security inspections at the gate itself, typically involving a full-body patdown (no inspection of the crotch area in the UK since this is illegal), a full luggage search, and a scan with chemical sniffers if these are available. I have undergone these only at Heathrow for US-bound flights, and I have never had water confiscated as part of them, but there have been reports of passengers being relieved of drinking water during similar inspections at east Asian airports like Singapore.
Generally, drinking water and bathrooms go together, so there is unlikely to be a last chance to replenish drinking water that is not also a last chance to visit the bathroom. But in my experience, it is more critical to board the plane with an adequate supply of drinking water than with an empty bladder. Planes have bathrooms, typically in the ratio of one bathroom per 30-40 passengers on a Boeing 777 in transatlantic revenue service, which is generally enough to accommodate multiple bathroom visits without long waits over an 8- to 12-hour flight. But if you board the plane without your own water supply, the cabin crew becomes your sole source of drinking water, and they are very stingy with it.