There is, somewhere back in the annals of rabbit raising history, a book which states that rabbits do not need to be offered water. This incorrect factoid surfaces every once in a while on various mailing lists or newsgroups. Originally, the “no water” diet that was recommended consisted almost entirely of fresh foods. In fact, rabbits who are eating a diet high in fresh foods do get much of their daily water intake from those foods and do not drink much water. It’s possible, even, that they received sufficient water for growth and breeding.
However, the diet recommended on these pages is not the same. Yes, it is high in fresh foods, but there is not nearly enough water in the foods to support a bunny. Rabbits should always have fresh water available. Water is critical to the health of the intestine and keeping the gut working to prevent ileus.
I prefer using bowls to bottles. In my experience rabbits drink more water when they are offered water in a bowl. Bowls take a little more work on the part of the caretaker, as they must be washed and cleaned much more frequently than bottles.
Many rabbits who are eating large amounts of vegetables will not drink very much. In most cases this is nothing to be concerned. If their drinking habits have changed, while there has been no change in their diet, then it is time to talk to a vet.
I know of a few cases where this has happened. Usually, it can be tracked to habit or boredom. Adding more frequent or different stimuli will normally distract bunny enough to decrease water intake. Excessive drinking can be a symptom of diabetes mellitus, but this typically is accompanied by excessive food intake and inability to maintain or gain weight.
In Conclusion
Rabbits should always be provided with fresh, clean water. In hot weather, ice water can help keep bunny cool. Water bowls seem to be easier for some rabbits to drink from, and they appear to prefer them to the typical water bottle.