True, but things in this universe are relatively standard. Whether on Earth or on an extrasolar planet, water is going to always be the same - 2 parts Hydrogen, 1 part Oxygen. Same with biological life, they'll always have some kind of genetic markup, something that makes it, "it".
That comparison just doesn't make sense. Water is water is water. One molecule of water looks the same as another. The same cannot be said of life forms. What if a ecosystem exists that is based on silicon chemistry rather than carbon chemistry? If you can't comprehend the potential for the different forms of life then I can't really help that.
Life didn't evolve on this planet the way it has because this is the current phase of evolution to a potential supremely evolved life form. But it is because the current life forms fill their niche the best way. It also doesn't mean that DNA is the only way to encode the blueprint for a life form but it evolved because of the building blocks and the conditions on Earth at the time it formed and reproduced were favorable. DNA is not necessarily a fundamental requirement for life on other planets, it is here but it is a evolved solution to the problem of reproduction. I would like to see an statement by any reputable scientist that says that DNA is a prerequisite for life (by this I mean extraterrestrial). They would need a method of reproduction but I can't see it being based on ours unless the theory of polyspermia holds true but even then that would just increases the chances of finding DNA based life rather than making it a certainty.