The question is not clear. Which DNA? Every water sample has DNA because it has microbial community. But if you are interested in specific DNA, you can use PCR reaction.
It depends a lot about what is the sense of your study and it's related with the sample size. The best approach, under my point of view, is define your sample size and use a ultracentrifuge. With rpm enough you can concentrate viruses particles. After that you can work with the pellet for a DNA purification and evaluation.
This will depend on your target DNA. But the first step will be to carry out DNA isolation using the right kit (EZNA water DNA kits (Omega, America), then proceed to the amplification procedure where primers and cycling conditions specific for your target DNA will be used.