Plastic bottles are generally designed and tested so they can be used safely for their intended purpose.
This means that if you want to reuse a plastic bottle, you should check that it's OK to do so by following any specific instructions given on the bottle. Or you could check with the manufacturer or supplier direct.
But, in general, if the condition of the bottle hasn't deteriorated and the bottle can be cleaned properly, it should be safe to refill it with water. You should wash drinks bottles thoroughly with hot soapy water, to stop any harmful germs growing. Bottles with cracks or dents in them mean they are harder to clean and remove germs.
If you're in any doubt, however, it's probably best not to reuse it.