Assume they are and make fresh water available at all times.
Also if your pet is small, note that dehydration in pets can come quickly with deadly consequences. Moderate dehydration in small pets or babies may require immediate medical attention.
Also note that dog dehydration is more likely than feline dehydration (cat dehydration) so be especially alert to your dog's symptoms.
Look for signs of dehydration in animals that aren't obvious like loss of appetite, irritability, fatigue and the like.
If you feed your pet dry foods, moisten with some water if possible.
For cats and dogs, you may even mix in a little pedialyte into their water to assure that they also get badly needed minerals in addition to fluids.
As a final tip, if your pet drinks from a bowl, make sure that bowl can't be tipped over, or provide several available bowls of water or provide a dripping water source like a faucet or hose.
This is especially important if you will be away for awhile.