Answer: Rising and Wildlife
Explanation: There are a few water sources that will rise, like oceans, bays, rivers and the like. So if you camp near a water source, there's a high chance that the water levels will rise, flooding the camp and yourself.
Another reason. You aren't the only organism that needs water, animals do as well. So if you camp near a water source, you also have a high chance you will run into aggressive and hostile animals. And there's no positive chance you'd be able to mitigate a territorial animal.