Bananas actually do the exact opposite of dehydrating you; they are able to treat diarrhea and keep your body from breaking down. In fact those who suffer from constipation are encouraged not to eat these fruits because they are known for contributing to the problem.
Those who are dehydrated should drink plenty of liquids as well as eat a banana or two so they can avoid any serious health problems as a result of getting dehydrated.
The best thing you can do is to put liquids and foods in your body that will prevent your body from getting dehydrated, especially if you are engaging in a very physical activity.